Greetings! I shall attempt to give my tuppence on this question, and I hope that other contributors with more specialised knowledge in the matter can delve into the rabbit holes I dig up a bit more. Firstly however, I would like to address some rather controversial claims you have put forward in the question.

Firstly, what exactly do you mean by "constant purges of the German army?". Is there a particular event or figure you can point to regarding how many general or field officers Hitler dismissed prior to the war's outbreak and in its course? The only event of considerable note that my past research has yielded is the Blomberg-Fritsch affair of 1938, which caused Hitler to gain full control of the Wehrmacht.

Secondly, I would like to correct your somewhat egregious statement here:

Authoritarian regimes always have incompetent militaries because personal loyalties are more important to a dictator than competence, while militaries of democracies have very competent militaries because personal loyalties don’t matter.

There is a lot more to an army's competence than the loyalty of its members. Quality of weapons and equipment, training, tactics, battlefield conditions, command structures, individual troop characteristics, size of the deployed force, and the list goes on for a while. I would really hazard against stating definitively that political loyalties are more important to success to every authoritarian state's armed forces. If we take that maxim to be a general rule, then the case of Nazi Germany's Wehrmacht in the opening stages of the Second World War or even the Soviet Union's Red Army in the later stages of WW2 disprove it. On the other hand, democratic armies are not guaranteed to be "very competent" because political loyalties are not a major concern. Churchill for his part, was notorious in his support or disdain for members of the Navy, Air Force, and Army, and often it was his backing (or lack thereof) which initiated command changes in entire theaters. He notably backed Admiral Andrew Cunningham of the Mediterranean Command when Sir Claude Auchinlek and Air Officer Arthur Tedder attempted to remove him from his post.^(1) How an army fights is based on far more than its loyalty to the regime, presidency, or state which commands it.

With that rather long preamble aside, let's get started.

Competent Commanders

"Where most states have an army, the Prussian army possesses a state." - Voltaire

The Wehrmacht of the Second World War was not a new military phenomenon in Germany, or in the Prussian state which had united the nation in 1871. It was the latest iteration in the German armed forces, and it carried with it all the soldierly traditions of a nation whose militarist origins have been written about by many historians. Even before Hitler came to power in 1933, the Reichswehr which had come about due to the Treaty of Versailles began to immediately eschew these militaristic hallmarks which had allowed the Deutsches Heer to maintain a war on two fronts from 1914-18. As Christopher Chant rather nicely states:

“The officers and men of the ‘hundred thousand army’ were taught to regard themselves as professionals– a hardcore force of experts, a functioning nucleus working for the future.”^(2)

It was this basis of training which saw the German army undertake massive leaps in research and development in the 1920s and 30s. Perhaps the most striking advantage which the army gained was the tactical doctrine of Auftragstaktik (mission-type tactics), in which the individual soldier was taught how to take their own initiative in battle situations, and placed independence of command on officers without having to constantly seek approval from superiors.^(3) Even as early as 1931, one finds this doctrine at the heart of the German military, with an actual extract from the 1931 Germany Army Training Directions reading:

‘the individual soldier must be educated so that he is able to accomplish his tasks in battle even if left to himself. He must know that he alone is responsible for his acts and failures’^(4)

Technologically, the Germans were also always abreast of the rapid developments in military technology, both within their corporations and in foreign nations. Most notable of these “foreign missions” was the German-Soviet Joint Tank development program, which lasted up until 1933 and is regarded historiographically as a major step in the development of what would later become the Blitzkrieg doctrine of the Wehrmacht.^(5) These pre-war efforts, accelerated with Hitler's shift towards expansionism in 1936 onwards (alongside the preparations of the German military-industrial state), ensured that by September 1st, 1939, the Wehrmacht had grown to become a potent force.^(6) More on how it utilised such potency in the next part.

When the war itself broke out, Hitler already had complete control of the Wehrmacht, by being the Commander-In-Chief of the Oberkommando der Wehrmacht, which was itself the top organization of the Heer (Army), Luftwaffe (Air Force), and Kriegsmarine (Navy) combined. In the early stages of the war and right up to its conclusion, the Army had in its ranks some of the most talented and capable commanders history has borne in such a short span of time: Panzer corps commanders Erwin Rommel, Heinz Guderian, Paul Ludwig Ewald von Kleist, Georg-Hans Reinhardt, and Erich Hoepner to name a few.^(7) These generals were often early proponents of mechanized warfare, and their tactics as leaders of the elite Panzergruppen and later Panzerarmee have been studied and applauded by military analysts/historians alike.

Even though these commanders and their subordinates knew that it was political loyalty which could (and often did) ultimately dictate who got the boot and who remained in the field, their professionalism and talent time and time again showed Hitler how invaluable they were to maintain. So to end the first bit of my response to your question, I’ll let Christopher Chant drive home the point on the Wehrmacht's leadership:

“In the last analysis it is safe to say that the combined armies of Great Britain, the British Dominions and Empire, the United States and the Soviet Union put together failed to produce so rich a crop of highly talented generals as boasted by the German army alone. The achievements of Hitler’s generals in the field were formidable enough. That they managed to achieve anything in the face of Hitler’s megalomania was positively astonishing.”^(8)

Coming tomorrow: The Lightning Strikes

Sources:

[1]: Lambert, Andrew D. Admirals: the Naval Commanders Who Made Britain Great. London: Faber and Faber, 2009.

[2]: Fritz, Stephen. The First Soldier: Hitler as Military Leader. S.l.: Yale University Press, 2018.

[3]: Chant, Christopher. Warfare and the Third Reich. New York: Barnes & Noble Books, 2000.

[4]: Quoted in Chant, Christopher. Warfare and the Third Reich.

[5]: Stein, George H. "Russo-German Military Collaboration: The Last Phase, 1933." Political Science Quarterly 77, no. 1 (1962): 54-71. Accessed January 18, 2021. https://www.jstor.org/stable/2146497. (JSTOR access required)

[6]: Parker, Robert Alexander Clarke. The Second World War: A Short History. Oxford: Oxford University Press, 2001.

[7]: Chant, Christopher. Warfare and the Third Reich.

[8]: Chant, Christopher. Warfare and the Third Reich.

In the previous comments I explored the role of the competent commanders, prewar preparations, emphasis on surprise, and the general inferiority of the Allied powers which led to the Wehrmacht's early successes in the 1939-40 period. We must now turn our gaze east, and to quote the Fuhrer who remained ever distrustful of his commanders (but acknowledged their brilliance):

"We terminate the endless German drive to the south and the west of Europe, and direct our gaze towards the lands in the East... If we talk about new soil and territory in Europe today, we can think primarily only of Russia and its vassal border states."

Again, a reminder that this response (in tandem with the others) is meant to serve as a general overview of the campaign in Russia from 1941 to 1942, as your question (rather thankfully) has narrowed the chronological scope down to those years. A more exhaustive analysis of the Wehrmacht's performance and the Red Army's initial failures requires further reading, hence I shall plug the AskHistorians reading list on the Eastern Front to help get you started on that venture if it interests you.

With all that said, let us now pick up where we left off and round off this trilogy of responses. It's been a most enjoyable journey, and I can only hope that more people find the overview here helpful (shameless preachy rhetoric there, but if I may be allowed it for brief moment).

Red Beard, Red Army, Red Flag

Similar sources to the comments above

The origins of Hitler's campaign of conquest shifting from the West to the East resides in several factors. First we have the rhetoric he presented in Mein Kampf and built up in subsequent publications/speeches, the idea of lebensraum (living space) for the greater German volk (people). Even before the Battle of Britain had gone into full swing, Hitler was already considering what to do with the USSR. Of course, he had technically entered Germany into a non-aggression Pact with the Soviets (the Molotov-Ribbentrop Pact) and that technically meant that Germany could not declare war on the Soviets, but Hitler had always expressed a desire for something to come out of Russia, and that "something" is described rather well by Chant below:

"Russia must be dismembered so that the ‘Slavic sub-humans’ might be put in their correct place as underlings of the master-race, so that communism might be stamped out as a threat to national-socialism, and so that the German race might possess the necessary Lebensraum or living space in the east, together with its enormous potential in foodstuffs and industrial raw materials, not to mention oil. As always, this last item was well to the front of all Hitler’s thinking."

But Britain was proving to be a tougher nut to crack than Hitler and his general had foreseen. The Luftwaffe were bombing British airfields (and later towns) day after day, but the RAF continued their stubborn resistance against the stream of bombers that flew out across the Channel from airbases in German-occupied France. This comment again will not go too far into the course and eventual consequences of that Battle on its own, so I highly recommend cementing at least some foundational understanding of the Luftwaffe’s failures against the RAF.

For the purpose of this response, the key consequence is that the Battle of Britain brought the Soviet Union to the forefront of Hitler’s concerns regarding the course of the war. On the one hand, he could choose to maintain co-operative relations with the Soviets, who for their part were rather willing to fulfil German demands for food and minerals. On the other however, by 1940 Roosevelt had decided that American resources were best directed to defend British independence, which posed a new threat on the horizon to Hitler. If the Red Army could be destroyed in a single campaign and the Soviet resources brought under German control, the Third Reich could face the eventual Anglo-American alliance with greater confidence and attritional resistance. However, this all rested on the key assumption: if the Soviets could be taken out in one campaign.

To that end, Hitler approved the plans for Operation Barbarossa (red beard, after a highly respected Emperor of the Holy Roman Empire) on December 13th, 1940. Preparations were to be completed by May 15th, 1941 and the main plan envisioned three powerful thrusts. More on that in a moment, because we must first take into account the sheer scale of this venture.

In the Low Countries, the Wehrmacht had deployed some 91 divisions over a front of about 965 kilometers (600 miles), against the might of the Red Army, they deployed 146 divisions, along a front of 2092 kilometers (about 1,300 miles) in length. The Red Army divisions facing them in June 1941 totaled some 150 divisions, though most were understrength and the whole Soviet armed forces was still suffering from something the original question has brought up: Stalin’s Purges.

It is not true that the Wehrmacht had suffered as equally by Hitler’s hand before the war as the Red Army has. The Blomberg-Fritsch affair I mentioned earlier stands out, but that only saw the removal of 2 commanders. By contrast, the Red Army’s top brass had been absolutely decimated between 1938-39. In the Red Army alone, the purges had eliminated: 3 out of 5 marshals (among them the renowned Mikhail Tukhachevsky), 50 out of 57 corps commanders, and 154 out of 186 divisional commanders. A new batch of officers and generals was being trained, but these men would take time to grasp the intricacies of mechanized warfare.

The Soviet equipment, on the other hand, was also a mixed bag. Though they had 20,000 tanks at the onset of the invasion, more than three-quarters were unserviceable. 1,500 KV-1s and T-34s (both of which were shockingly superior to any German panzers of the time and impenetrable to all but the heaviest field guns) had been deployed, but these formations lacked radios and were inexperienced. The Soviet Air Forces had about 8,000 planes, but most were old designs and crewed by pilots who had yet to face the battle-hardened Luftwaffe. Soviet artillery on the other hand, was a formidable enemy if encountered, though in June 1941 they were still suffering from the purges as well.

Barbarossa was formed of a three-pronged thrust into the Soviet heartlands, striking at key cities such as Leningrad, Moscow, and Kiev, as well as seizing the Caucasus oilfields in the south. The first target was entrusted to generalfeldmarschall Wilhelm Ritter von Leeb’s Heeresgruppe Nord (Army Group North), formed of six mobile and twenty-three infantry divisions. Kiev was entrusted to generalfeldmarschall Gerd von Rundstedt’s Heeresgruppe Sud (Army Group Center), formed of eight mobile and thirty-three infantry divisions (among them some Romanian divisions). The prize of the Soviet capital however, fell to generalfeldmarschall Fedor von Bock’s Heeresgruppe Mitte (Army Group Centre), formed of fifteen mobile divisions and thirty five infantry divisions. Among the troops under Bock’s command were the 2nd and 3rd Panzergruppen of Erich von Manstein and Hermann Hoth, two of the most battle-hardened and well-equipped contingents in the entire force. The general tactics were similar to those which had worked so well in the West: mass encirclements of Soviet forces enabling swift strikes towards key points. However, the critical factor here, was supply. The encirclements were planned to take place as close as possible to the Western border as possible, enabling the resupply and reinforcement of the various army groups to sustain the campaign. It was this critical factor which soon shot the bolt of the entire Russian campaign.

On June 22nd, after a crucial month’s delay at Hitler’s insistence to deal with Yugoslavia and Greece, the operation was launched. In the first few weeks, all three Army Groups made spectacular progress. By July 13th, some 4 million Red Army troops had been killed, wounded, or captured. The Wehrmacht had pushed the borders of the Third Reich as much as 800 kilometers (600 miles) further. Massive encirclements of hundreds of thousands of troops had taken place around Bialystok, Minsk, Kiev, Kharkov, and Vyazma (not including the countless smaller encirclements of sub-division formations). Chief of staff general Franz Halder remarked in his diary on July 3rd:

“It is thus [in light of the success of the invasion] probably no overstatement to say that the Russian Campaign has been won in the space of two weeks. [However], the sheer geographical vastness of the country and the stubbornness of the resistance, which is carried on with all means, will claim our efforts for many more weeks to come.”

The Lightning Strikes

In the previous comment I touched on how the interwar years were a hive of activity, R&D, and general preparation for the Germany Army (even before the ascent of Hitler as a dictator in 1933). Though I highly recommend you look more into specific historical analyses of the German army’s command strengths (especially if you are particularly interested in a certain Panzerarmee or infantry commander), the key takeaway from Part 1 was that despite the undeniable importance of loyalty to the Nazi regime and Hitler as Fuhrer, the Prussian sense of militarism had already been distilled into an incredibly competent group of generals who oversaw the early years of the war. This part of the response will go more in-depth (though by no means is it complete or exhaustive in that effort) on the tactics that the Wehrmacht employed, and why that enabled it to win a swathe of successes in the 1939-41 campaigns.

Firstly however, we must acknowledge that elephant in the room. The very heading of this part is a nod to that inescapable word when studying the German military during the Second World War: Blitzkrieg (Lightning War). Most textbooks, tertiary sources, and even documentaries (though “pop-history” ones at that) ascribe the success of the Wehrmacht to this “radical” new style of warfare, which emphasized rapid and sudden attacks by concentrated formations on a weak point, followed by envelopment of the opposing forces and eventual annihilation of them.^(1) This entire concept however, is a myth. It’s now widely known in historiographical circles (even mores after the writings of Richard Overy and Williamson Murray in the 1970s). The term itself is actually an Allied creation, in that it was actually the Anglo-American Press which coined the term and used it with such frequency in the immediate post-war historiography. J.P Harris on this odd paradox:

“Those who made the Third Reich’s military plans and organized its war economy appear rarely, if ever, to have employed the term Blitzkrieg in official documents. Nor was any equivalent official term used which carried all or even most of the meanings and connotations with which Blitzkrieg has been endowed by English-language writers.”^(2)

Without going into too much of the debate here, the essence of it all is that Blitzkrieg was not the concerted effort of mechanized troops and Panzers which is often pictured when hearing the word. Instead, Blitzkrieg (if one can even use the term) referred to the Germany Army’s preference for fast, and short campaigns, which during the Second World War was launched though the use of Panzergruppen and the Luftwaffe’s “Flying Artillery”.^(3) So, let’s take a closer look at the Poland Campaign and how this tactic really unfolded on the battlefield.

The Polish Army in September 1939 consisted of thirty regular infantry divisions, ten reserve divisions, eleven brigades of horsed cavalry and one armored brigade.^(4) Facing them from Germany and East Prussia were fifty-five German divisions. Twenty-four of them were infantry divisions in the so-called “first-wave”, regular soldiers and the best reservists. Fifteen were infantry of regular cadres and younger reservists, whilst the sixteen divisions were composed of six new Panzergruppen and ten fully mechanized infantry.4 Importantly however, the Germans had overwhelming superiority in the air, with over 2,000 modern aircraft crewed by well-trained Luftwaffe pilots decimating the 600 obsolete Polish aircraft.^(5)

In truth, the German campaign was not spearheaded by the lightning-fast and precise Blitzkrieg strikes which is often detailed. Instead, the Panzers were deployed in a dispersed manner, aiming to overrun key communication points, supply lines, and defenses.^(6) Further, the use of East Prussia as an actual launch point for the invasion was a stroke of genius, as it not only gave the Polish defenders more to worry about, but presented a golden opportunity for encirclements. To this regard, the Wehrmacht assigned the 3rd and 4th Armies of Generaloberst Feodor von Bock’s Heeresgruppe Nord (Army Group North) the duty of the East Prussia sector. The 4th Army would attack from East Germany into the Danzig Corridor, linking up with the 4th Army (which would itself also split into two to attack various other cities from East Prussia).^(7) The campaign was a complete success, and all resistance had crumbled by the 17th (the same date the USSR launched its invasion of Eastern Poland).^(8) Stephen Fritz on the success of this first Wehrmacht offensive.

“Although their tanks compared favorably with German Panzer Is and IIs, the Poles had few of them and, most damaging, were completely deficient in terms of number and quality of aircraft. The Poles were also hurt by Allied pressure to delay their mobilization so as not to provoke a German attack, an action that seemed to corroborate Hitler’s perception that the British, especially, still hoped to avoid war.”^(9)

Against France and Britain in the 1940 campaigns however, the Germans would not be able to take advantage of numerical or even materiel superiority. The French fielded the equivalent of 91 divisions in the Spring of 1940, and their tanks (namely the H-35, H-39s, and Char-Bs) were in many respects the equivalent (and in some ways better) than the already aging Panzer I and IIs that were still in some of the Panzergruppen.^(10) The British for their part, contributed ten infantry divisions in the British Expeditionary Force, which was for the most under equipped and poorly trained.^(11) The Luftwaffe once again outnumbered and outgunned the combined British, French, Belgian, and Dutch air forces and the presence of radio in every element of the now elite Panzer divisions (as opposed to the lack of it in the Anglo-French ones) meant that the Germans had a slight upper edge troop-wise.^(12)

The real competence and brilliance of the Wehrmacht in this campaign however (known as Fall Gelb, or Case Yellow), was in choosing where and when to strike. You have probably read already about how Manstein’s Panzers bypassed the Maginot Line by way of the Ardennes Forest and surprised the Allied forces, who (some less reputable tertiary sources claim), had not expected such an attack. This final note is egregiously wrong: the Allied planners had already envisioned such an attack through Belgium, and the French high command had discussed in detail their Army’s plans for such a maneuver. General Gamelin, the supreme Allied commander, pushed for what the French called “Plan D”, in which a German attack would be met with a defensive line on the River Dyle from Wavre to Louvain and Antwerp, and then onto a line on the Meuse River from Namur to Sedan.^(13)

The Wehrmacht plan consisted of a two-pronged assault involving Army Group A and B (under the command of Gerd von Rundstedt and Feodor von Bock respectively). Army Group A was made the larger army in an insightful decision by Erich von Manstein and Rundstedt (wihich Hitler supported), as this was the group which contained the armoured spearhead of seven Panzer and three motorized divisions (alongside other formations, it totaled some 44 divisions).^(14) The Germans achieved surprise with their attack, and the French centralization of Command, as well as general incompetence by its commanders, meant that the campaign progressed with unexpected speed. R.A.C Parker on Fall Gelb:

“In war between comparable forces victory goes to the side which suffers fewer delays and confusions and in which the chain of command is more lucid and effective. The best French troops and their equipment and morale were fully equal to those of the best German troops. They were defeated because too often they were not in the right place at the right time.”^(15)

So in addition to its use of armored divisions and rapid maneuvers, the German Army made such short work of the 1939-40 campaigns against roughly equal Allied armies due to its superior command structure (or lack thereof, recall the Auftragstaktik from the previous part) and mastering the art of surprise. Though behind the scenes Hitler had quarreled with many commanders in the lead-up to these campaigns, he could not deny their effectiveness and professional merit by the time Paris had fallen. Competence and brilliance in the Wehrmacht came before blind political loyalty, and the generals who scythed their way through Poland, France, Luxembourg, Belgium, and the Netherlands were exemplary at the former, often making up for their deficiencies in the latter.

Coming tomorrow: Red Beard, Red Army, Red Flag

Edit: Part 3 has been delayed for tomorrow, due to my heavy workload today. Sorry about the wait!

Just to add my two cents:

Authoritarian regimes always have incompetent militaries because personal loyalties are more important to a dictator than competence, while militaries of democracies have very competent militaries because personal loyalties don’t matter.

While u/Starwarsnerd has already addressed it, I would like to get back to this point: as any other as any other black-and-white notion about history this one is very far from truth: The French army of the Third Republic since it creation in 1871 till its fall in 1940 was plagued by the problem of loyalty-based vs merit-based promotions. The civil government of the republic was constantly in fear of some form of Bonapartism military coup toppling the democratic government and establishing the Third Empire, while the constant threat of the another German aggression and the revanchism forced the government to maintain potent and powerful military. This resulted in very complicated and troublesome relationship of government and army of the Republic. The officer cadre of the army became heavily factionalised into numerous groups of Republicans, Patriots and Careerists and promotion of young officer almost always required the patronage of some higher-up civil or military official which even further contributed to the formation of various cliques around the most influential officials. This "ghost of Bonapartism" did not wane even after Genera Boulanger's attempt to seize power has failed.

This problem has only worsened in the interwar period. The pacifist sentiment of the lost generation combined with the global economic crisis has forced the government to be very stingy about the military budget. Simultaneously the cases of militaries of Italy, Spain and Germany actively supporting the nationalist anti-democratic coups combined with the growing nationalist movement in France itself has renewed the old fear of Bonapartism. The overall political and ministerial instability of the Third Republic has only exacerbated the importance of military leadership’s loyalty. The question of the size of officer and professional NCO cadre became a political one closely controlled by the parliament and its enlargement was always very heavily opposed by the left politicians. The government has took proactive steps in preventing the formation of professional army: the conscription service was shortened in order to prevent the formation of ties between the commanders and their soldiers while the field training and manoeuvres were heavily restricted. De Gaulle's 1934 Towards a Professional Army was criticized by the Left who believed that professional army would inevitably take down the democratic government and seize power for the fascists.

All of this resulted in French officers being in general very incompetent and sometimes even comically inept at the simple communications and manoeuvring because they never had commanded their units in the field before the war. So, to conclude we have an example of the army of the probably most democratic country of Europe being endemically plagued by the same problem you've attributed to the dictatorships. Another example to prove this was the Spanish civil war: both Nationalist and republican armies included forces from a wide spectrum of political allegiances, but the Francists almost never had a problem of loyalty and trust in their commanders during the war while for the Republicans it was a constant factor they had to keep in mind.