I think the first thing that needs to be cleared up here is that Germany famously didn't win but instead lost both World Wars. I'll focus on WWII here since that is more my expertise and when considering WWI it is also important to think of both the Austro-Hungarians and Ottomans as additional war participants.

The second thing that is important here is that modern 20th century large scale war is really a question of the very large scale administration of resources and logistics. Germany had in 1939 (including the Sudeten and Austrian territories) a population of 87 million people, all of which concentrated in their core European territories (in comparison to e.g. France which had an imperial population of 114 million but with e.g. 14 million being the populous of French Equatorial Africa or 13 million of French West Africa and European continental France sporting a population of 42 million people). But while these numbers do play a role, what is even more important is how to administer this population. What I mean by that is that when you put a soldier in the field, this soldier needs a gun, ammunition and something to eat as well as a way to put him in the field. These things in turn need to be created in factories and on farms and delivered to the field. Generally, ever person you put in the field is going to miss at home in the factories and on the farms while at the same time they increase your need for war material. Basically, modern war is an exercise in large scale logistics, planning, and administration of resources more than anything else because both strategic and even tactical decisions will always depend upon these factors and they also shape what kind of political and strategic goals a military has.

I'll get more specific on German policy shortly but in terms of numbers, in WWII a total of 17.8 million German men served in the Wehrmacht – Army, Navy, Air Force and Waffen-SS – in the years between 1939 and 1945. That represents about 20% of the total German population or about 40% of the total male population and an even larger percentage of the total male eligible population (between 17 and 45, being physically able to serve in the Wehrmacht). That is a rather large percentage overall and represents a whole lot of people who had to be replaced in their civilian jobs, from running and working in factories, farms, driving public transportation, policing the streets and so on and so forth. This was possible due to mainly two factors: Rotation and exploitation.

Rotation is rather easy: Not all of the 18 million soldiers of the Wehrmacht served at the same time. People could leave the service after a specific campaign was over and the need for manpower was reduced to be called up again later or to spend the rest of the war working their former or new civilian jobs. This becomes evident from the numbers of in-service soldiers of the Wehrmacht over time: While during the initial period of the war until the defeat of France the Wehrmacht saw at least 4 million Wehrmacht soldiers in service, at least a million of them was released from service after the campaign was over and it took until the spring of 1941 that a new mobilization took place for both the Balkan and especially the Soviet campaign. Basically, not all 18 million Wehrmacht soldiers of WWII saw service at the same time and the maximum number of soldiers in Wehrmacht service at the height of numbers was 12 million people.

This is fairly standard for modern armies but the really crucial factor for the Germans to be able to sustain their fighting force while at the same time feeding their own population and keeping up with manufacturing war material was exploitation. Exploitation of their own workers as well as massive exploitation on every level of occupied and controlled territories and its people.

Germany, on an economic level, effectively went to war in 1936. That was the year, the Reich leadership decided to pursue to a policy that favored heavy investment in war material vs. consumer goods since both at the same time could simply be not sustained with the labor force and resources they had available. The way this worked was the self-organization of businesses in the frame work of the Four Year Plan with business operating basically under a war economy in peace time for profit. Because of massive state investments, e.g. business that had manufactured fridges before would change their production line to something more needed for the war economy such as rifle or tank parts and so forth.

At the same time, state imposed restrictions on workers got more severe than ever before. Unions had been dissolved anyways since 1933, there was no standard 8 work day anymore, and people became unable to change employers without previous authorization of the labor agency. In fact, by 1939 the labor agency became able to place people in factories and workplaces where they were most needed for the war economy regardless of the fact if they wanted to work there or not.

But even more crucially, once the war started, labor force became a strategic war goal. The first German civilian agency that started to operate in Polish territory was the German labor agency and by October 15, 1939 already 126 offices of the German labor agency had opened their doors in Poland and had started organizing Polish laborers to be shipped to Germany to replace the men in service of the Wehrmacht, in the beginning crucially in agriculture.

In the vast majority of all cases, these Polish laborers didn't go to Germany voluntarily but as part of a vast system of forced labor. A system so vast, German historian Ulrich Herbert termed it the largest system of unfree labor since the international end of slavery in the 19th century. Using methods ranging from economic pressure to sheer violence (burning down villages, raiding trams and marketplaces), the German system of forced labor encompassed in the years from 1939 to 1945 about 12 million people in Europe. At its height in August 1944, 7 million foreign forced laborers were in Germany representing about a quarter of the total labor force in Germany. And that fails to include the forced labor in Concentration Camps – which became one of the primary purposes of the Concentration Camp system overall starting in 1938 and escalating over the course of the war into such things as the IG Farben Buna Works maintaing what was basically their own concentration camp in form of Auschwitz III Monowitz.

Living under horrible conditions in the Reich (forced laborers had to wear their own badges, were not allowed to use public transport or visit sites of entertainment, had a 12 hour work day, Russian forced laborers getting their own specially made bread mostly from sawdust etc.), the exploitation of occupied territories didn't stop at the labor force. The other important factor of how Germany was able to field so many soldiers was the ruthless exploitation of occupied and controlled territory for foodstuffs.

Feeding the German populace throughout the war was one of the highest priorities of the Reich leadership for the traumatic memory of the hunger winter of 1917 and the ensuing revolution in 1918 still lingered large among them. The only way for them to adequately solve this problem was in letting other territories starve. While the hunger plan against the Soviets was never fully implemented, famine still swept the territories occupied by the Wehrmacht. Greece went through mass famine and starvation due to the German and Italian occupation that could only be alleviated by lifting the Allied blockade against Greece. Still, at least 30.000 Greeks starved because primary the Germans siphoned all the foodstuff out of the country to feed the German population at levels they deemed acceptable.

It was only this sort of exploitation that enable the Germans to filed such an army and to continue fighting for as long as they did. The Nazi New Order and the war the Nazis was fought was one build on ripping millions of people from their homes, shipping them to Germany and forcing them to work and letting thousands upon thousands of people in Europe starve. The war they fought was always one of annihilation, exploitation and cruelty and the order the brought was one of famine, starvation, slavery, and death.

Sources:

• Adam Tooze: The Wages of Destruction.

• Mark Mazower: Hitler's Empire: Nazi Rule in Occupied Europe.

• Mark Mazower: Hitler's Greece.