TL;DR: Painful but necessary, and productive

Part One: Chantilly

In November, 1915, Joseph Joffre, the Commander of the French Army and essentially the Allied generalissimo, called a meeting at Chantilly, to decide on Allied strategy for 1916; Italy, France, Russia and Britain were all represented here.

1915 had been abysmal to say the least: Gallipoli and Salonika were blind alleys, the French Offensives in the west had been unable to prevent the Germans from concentrating forces in the east, Serbia and Montenegro had been conquered, Bulgaria joined the Central Powers, and Russia had suffered c. 2 million casualties in the year. What was needed was a unified strategy to coordinate efforts on all fronts, and push the Austro-Hungarians and Germans to the breaking point. Once they were out, all else would follow.

The General Allied Offensive would comprise an Anglo-French assault on the Somme, the one area on the western front where the British and French lines met, and thus could combine their efforts. Three French Armies and ONE British army would attack, the concept being one of 'continuous battle': extensive use of artillery to smash the enemy's lines, accompanied by a break-in of the initial positions, followed by a continuous series of attacks, to force the Germans out of their successive positions. The aim would be to bend the German lines until, hopefully, they broke. Break through or not, this offensive would have the effect of inflicting serious losses on the enemy, and of tying down divisions in the west.

Italian forces would continue their advance against the Carso Plateau, aiming for Gorizia, while the Russian Southwest front, under Alexei Brusilov, would aim to tear a hole in the Austro-Hungarian Front, driving them back to Galicia.

The goal then was strategic attrition and overreach of the enemy's forces, through the combination of allied offensives on all fronts. Romania also seemed poised to join the Allies, and so this factored into plans in the east.

Part Two: Verdun

Unfortunately, the Germans struck first. Whereas the Allies aimed to combine their offensives, the Central Powers had different ideas. Conrad von Hotzendorff and Erich von Falkenhayn, the Austrian and German chiefs of staff respectively, each had their own pet projects: The Italian Front and the Western Front.

Falkenhayn's plan was an offensive against Verdun, the Fortress city located in a salient in the west. Success here would be two fold:

A) Taking the city would give the Germans control of the Meuse, and likely force an abandonment of the salient, shortening the western front.

B) if the Germans could simply take the ring of outer forts, and seize the bluffs overlooking the city, it could be brought under heavy artillery fire, and from their defensible positions the German Fifth Army could 'Bleed the French White'.

The Austro-Hungarian aim was an offensive against the Italians. Falkenhayn (fatefully) okayed it, and both sides went off on divergent, separate campaigns. The Austrian position in the East was thus weakened, while Falkenhayn expected to crush a British 'spoiling' attack on the Somme with the German Sixth Army, poised for a counter offensive between the Somme and Arras.

The Germans (like Han solo) shot first, and the Allied plans had to change in the west, as the French army was sucked into bloody fighting around the 'Meuse Mill'

Part Three: Brusilov and the lead up to the Somme

The French armies at Verdun came under Phillipe Petain's leadership, who sent increasingly desperate messages in June, as Fort Vaux and then Souville fell to German attack. While Joffre and General Douglas Haig, the commander of the BEF, hashed out strategy, the Russians made their move.

By early June, c. the 1st, Conrad's offensive in Italy had stalled, and by June 11th, a brief pause was allowed at Verdun (small consolation to the French!). In the First week of June, the Russian Southwest Front struck in Galicia. By September, Russian troops would be at the Carpathians as in 1914.

Alexei Brusilov had been given priority for ammo. and guns, and had the elite Guards Army as a reserve. He struck the Austrian lines around Dubno, pushing on to Lutsk, while further south the Austrian lines unravelled. Losses in the first week for the Russians were c. 490 000 casualties, Austro-Hungarian losses in the first 12 days were c. 280 000 casualties.

By the time the offensive reached its high water mark in August, and ground to a halt in September, Austro Hungarian losses were about 1 million, a quarter of that prisoners. Russian losses were about 1.5-2 million, perhaps half of them killed. Romania entered the war, and the Austro-Hungarians were forced to: A) accept German forces and officers in many of their higher units; and B) accept German command on the Eastern Front. Conrad was sacked, and Falkenhayn was given the boot for Romania's entry.

In France, events on the Somme were unfolding...

Part Four: Planning the Somme

When he took over the BEF from John French, Haig had been an army commander in charge of 100's of thousands. In July 1916, he commanded an Army group (essentially) of 5 Armies: the largest organizational body in the British Empire outside London and the Metropolitan Area!

He was in charge of a force that had not yet taken a leading role, having only played wingman for the French at Neuve Chappelle, Loos and Aubers Ridge. British industry was still going through growing pains, while the professional regulars and semi-professional Territorials of the old BEF were giving way to the volunteers of the 'Pals Battalions' and soon the Conscripts.

He would have liked to wait until 1917, when more of these new 'Tanks' would be available, his soldiers would be better trained, and the new doctrines developed from the experience of 1915 could be fully instilled. With regards to the latter, as Paddy Griffith notes in his book on BEF Tactics, it was only AFTER the Somme that an effective doctrine would be instilled, and it sure as hell showed at Arras and Vimy Ridge!

Now, in June, Haig was being informed that rather than attack in August with one British Army in the supporting role, and 2-3 French Armies leading, the BEF would take on the MAJOR role in the offensive, with ONE army, and ONE French Army supporting! Haig knew his men needed more time, but he also knew that was time that the FRENCH might not have. Britain was the JUNIOR coalition member on the Western Front, thus Haig was obliged to carry out Joffre and Ferdinand Foch's instructions.

The men chosen to lead the Somme operations were Henry Rawlinson, GOC 4th army, and Emile Fayolle, commander of French 6th Army. Both had initially similar plans, use heavy artillery bombardments to smash the enemy's defences, and take each position one at a time: 'Bite-and-hold'.

However, Haig had different plans, and this stemmed from it being his DUTY to factor in Strategic goals, and Rawlinson's JOB of focusing on tactical matters. The 'Operational' level of warfare wasn't really understood or acknowledged at this time; it would take WWII to cement it. Rawlinson's original plan was very small, and did not reflect the enlarged role of the BEF. The Front line was expanded to 20 km (Rawlinson originally planned for 15), and there would now be a role for the British cavalry.

Remember, this WAS Haig's job. If the opportunity presented itself, the British cavalry should be on standby to advance on the German positions. Moreover, Haig's idea was pretty ingenious: the cavalry divisions of 4th Army were split up, regiments given to each attacking infantry division. They were instructed to cooperate with the infantry, almost like combined arms battlegroups, and move on the German second position IF THE COMMANDER FELT THIS WAS FEASIBLE!

To accommodate this, Haig had the artillery bombardment extended to the second position, north of the Albert-Bapaume road, where Rawlinson's main effort was to be made. Beyond this, everything else was Rawlinson: the decision to spread the infantry across the ENTIRE front, because he expected no breakthrough, so why develop a mass in one area? Rawlinson spread the Artillery across the entire front, rather than amass concentrations of guns against the most heavily fortified positions. It was Rawlinson who issued the recommendations that infantry should advance at a slow, steady pace: despite the popular image of the 1st day, this was virtually ignored entirely.

It was Rawlinson who also got Haig to agree to a lengthy, methodical bombardment; Haig wanted a short, hurricane bombardment, but knew this was unfeasible.

The British bombardment faced major issues: unlike the French, it was spread across 20 km, not 10. The French had roughly the same amount of Guns as the British, c. 1400, but had MORE heavy guns. French artillery observers, on the southern part of the front, had a better view than their British counterparts north of the Albert-Bapaume road. They were also more experienced, positioning themselves closer to the lines. The French guns were of a higher quality than the British, and fired more HE shells, and better quality HE shells at that! They were ONLY shelling the first line, whereas British artillery had to hit both lines, but this probably did not disperse their fire as much as most think.

The British plan, now, was to contribute to the Allied General Offensive by wearing down the enemy's forces, and to relieve the pressure on Verdun, and KEEP IT off Verdun. They would break into the enemy's lines and capture the Pozieres ridge, along with other areas of high ground nearby, aiming at Bapaume and Peronne. Depending on how events went, the Anglo-French would either dig-in in favourable positions for 1917, OR, if the enemy DID break, the British Reserve Army under Gough could role up the German lines to Cambrai; it ALL depended, and this Haig knew, on the reaction of the enemy, and the course of the battle. On July, 1st, 1916, The Battle of the Somme began...

Part 5: The Somme begins

On July 1st, soldiers of the French 6th Army went over the top. They moved in small groups, advancing under artillery fire ad followed by 'trench sweepers' to clear enemy positions with grenades. By the end of the day, the entire German line south of the river was unhinged: the 'Poilu' were ecstatic!

Further north, between the river and the Albert-Bapaume road, fighting was bloody but decisive: XIII and XV Corps of 4th Army took the first line, and inflicted many losses on the demoralized enemy. Moreover, positions here were poor compared to the north, while the first line had been shelled heavily here; the second line was not a goal in this area.

In the north, things were very different. As Trevor Wilson and Robin Prior have demonstrated in their book The Somme, the imagfe of weighed down tommies walking towards death is a myth. Most Battalions crawled into no man's land and rushed the line. The 36th Ulster Division took the German positions around he Schwaben and Stuff redoubts; however, they failed to properly clear their rear areas, and were driven out by nightfall. Elsewhere, the attacks failed, not so much because of machine gun fire per se, as because of uncut wire, machine guns, and unsilenced artillery working together. The north, also, was where the Germans made their main effort of defence; the south was a stunning success.

In light of the terrible results in the north, Haig placed Hubert Gough's Reserve Army in charge of the effort north of the Albert-Bapaume Road, while Rawlinson reoriented his forces south. In the week that followed, the British and their French Allies steadily drove the enemy back. On July 12-14th, the British attacked Bazentin Ridge, assaulting at dawn after a swift, crushing barrage, and succeeded in taking the ridge. British Indian Cavalry successfully entered High Wood, but the infantry did not reinforce them, so they were withdrawn.

From July to August, the fighting took a form heavily criticized today: actions involving Brigades and Battalions fighting piecemeal battles, taking Pozieres, High Wood and other position at great cost. Despite this, the British AND French found themselves reduced to this effort by difficult supply lines, poor weather, and concerns with the enemy: they DID try to organize broad front attacks, but theses were difficult to coordinate, took long to prepare, and were no guarantee of success (one in August was met with disaster). The more time they took to prepare, the more time the GERMANS had to prepare a counter-attack, and build up their defences. Broad front assaults dispersed enemy strength and firepower, but did so to ALLIED strength and firepower. They needed to keep up the pressure on the enemy, and the German response was little better: according to the commander of the German 1st Army, Fritz von Below, there were to be NO retreats; "the enemy must advance forwards over piles of corpses" (roughly).

By September, however, the British and French were ready for a renewed frontal effort, with the French general assigned to coordinate the Somme battle, Ferdinand Foch, having developed an operational method: rather than try to coordinate an assault at the same time, which proved almost impossible, the British and French would attack one after the other, each undermining German positions opposite the other.

The stage was set for the 'Big Battle Days' (Grosskampftage) of September, as Ludendorff referred to them.

Part Six: The end on the Somme

The British attacked at Flers-Courcelettes, aided by tanks and the Canadian Corps, the French around Ginchy and Bouchavesnes. German losses in September were over 100 000, and the attacks brought them near to the breaking point. French attacks placed allied lines across the Bapaume-Peronne road, thus severing German lateral communications, while both cities were in striking range of Allied assaults. German positions hardened however, as there was a change of command, with Hindenburg and Ludendorff now in charge of the German Army and allowing a more flexible defence.

By October and November, the last gruelling months of the battle, the allies had reduced their aims to attainable, tactical objectives, to improve their positions. In late September, Ivor Maxse's 18th division captured the Thiepval Ridge (the site of the memorial to the Missing on the Somme) with the aid of tanks. By November, Beaumont-Hamel had fallen, and the heights of the Ancre river were in British hands. The Offensive was shut down: The Somme was over... for now.

In the air, the British enjoyed fairly unquestionable success; the Royal Flying Corps, under Hugh Trenchard, maintained air superiority, and allowed for British artillery to dominate their German counterparts. In many ways, the Somme campaign proved to be the first, all-arms battle.

Part 7: Are we done yet? Almost!

What DID the Somme do?

Well, it took back more ground than any Allied offensive since the Marne; it took key pieces of high ground around Bapaume and Peronne, forcing the Germans to abandon considerable territory and fall back to the Hindenburg Line in 1917.

It proved to be the BEF's trial by fire. Haig, Gough and Rawlinson gained much needed experience in leading a major operation, while Foch and Fayolle demonstrated just what the French Army could do, making remarkable achievements, which have sadly been largely ignored in favour of Verdun.

Britain had many skilled Corps commanders emerge during this battle, Henry Horne, Julian Byng, Claude Jacob, Cavan, Walter Congreve, and many other commanders who distinguished themselves, such as Ivor Maxse, the "liberator of Thiepval". British troops gained much needed experience, and learned the hard way how to operate as units in battle. The Royal artillery would emerge from this battle a feared organization, steadily mastering counter-battery, and exerting great effect on the battlefield. From this battle would emerge SS 143 and SS 135, new training instructions for British platoons and divisions, that would lay the stage for the development of the BEF into an all-arms force. The RFC demonstrated the importance of airpower in conflict, and the Tank showed a glimpse of the future.

On July 11th, operations were suspended indefinitely at Verdun, allowing breathing room for the French to regroup and, under Robert Nivelle's command, counter attack, taking back all the territory they lost. The planned counter attack by 6th Army against the British was nixed, the German divisions being fed into the meat-grinder on the Somme. It contributed to the Allied General Offensive, although there were mixed results over all.

Romania's decision to enter the war was foolish. Maybe in 1914 or 15, but now it was surrounded by Bulgaria and Austria-Hungary. After a brief offensive into Transylvania, the Austro-Hungarian and Bulgarian Armies, aided by c. 8 German divisions scrounged up from the East and a Turkish army corps, pushed them back, occupying Dobruja and Wallachia. The Central Powers gained access to Romanian grain, coal, timber and oil among other things, getting them through the harsh, 'rutabaga winter' (NOT Turnip Winter) of 1916-17; they also gained a longer frontline, and more territory to occupy.

In August-September, the Germans under Hindenburg raised c. 33 new divisions, by breaking up or splitting existing ones, and combing out the Landwehr (national guard), Landsturm (elderly reservists), convalescent wards, and factories; 'Germany's last hope' as they were referred to, were essentially the equivalent of the Volksgrenadier divisions of WWII.

Martin Middlebrook makes an important point in his book on the First Day of the Somme that it really was 'the point of no return'. British losses rose greatly after July 1st, as did British debt, and the expansion of British industry and the British state. Lord Kitchener, killed before the Somme on his way to Russia, was right when he said that for Britain "the real war wouldn't start till 1916." The British were in the fight now, there was no going back. The BEF's learning process accelerated greatly, and it now had the confidence to take on Europe's best army: and Win!

Losses, however, were indeed high. The best number for British casualties comes from Wilfrid Miles' second part of the British Official History: 419 654, of which 95 675 were killed or missing; a casualty rate of c. 2976 for the 141 days of the Somme. French Official Casualties were 154 446 for 6th Army, 48 131 for 10th Army, for a total of 202 577, 50 756 of which were killed or missing (1436 casualties per day of the Somme campaign). Compared to official French losses at Verdun, which were about 373 231. The French lost 54% of what they lost at Verdun in 10/11 months, at the Somme in c. 4 months.

German losses are, controversial to say the least. This is because of the different ways in which the Germans calculated casualties, often not including the sick and lightly wounded. James H. McRandle and James Quirk calculated German losses at Verdun as 373 882. For me, the most reputable numbers on the Somme come from Miles of the Official British History (1938), and German historian H. L. Wendt, in the 1931 history "Verdun 1916 Die Angriffe Falkenhayns im Maasgebiet mit Richtung auf Verdun als strategisches Problem". They estimate 465 181 and 434 500 Killed, Wounded and Missing in Action respectively; between 100 and 160 000 were killed and missing. Adding German prisoners, 41 605 taken by the French and 31 396 taken by the British, that puts estimated total losses at between c. 500 000 and 530 000; the official German number, and the number given by Napoleonic War historian Charles Oman, who was assigned during the war to compile a number based on German newspaper casualty lists. That leaves a casualty rate of c. 3500 to 3800 for the 141 days of the Somme. In total, German losses in 1916 amounted to 1.4 million.

52 British, 48 French and 95.5 German divisions saw service on the Somme, with typically 50 division on either side facing each other from mid-July to November. In total, German losses were less than Anglo-French, but the Anglo-French could accept these losses without GREAT difficulty; for the Germans, the losses were sorely felt. What remained of the professional corps of their western armies had been torn apart by the Somme, and Verdun, a terrible price for Germany to pay.

The Offensives forced the Germans to overreach themselves; the 1917 and soon 1918 classes were already being called up and now every German male between 17 and 60 could be called up for industrial or military service. Austria-Hungary could 'look forward' to a future either of German vassaldom, or dismemberment by the Allies, their army having been completely gutted.

To finish off, I'll leave you with three quotes, one British, two German:

"The last day of a year of indecisive fighting. Verdun, Somme, Greece and Rumania all indecisive, both sides claiming victory; on the whole victory inclining to us, and the final decision brought nearer." General Henry Wilson, 1917.

"The Battle of the Somme was the Muddy Grave of the German Army, as well as of our confidence in German supremacy, crushed by British industry and its shells. The German supreme command which entered the war with great advantage was beaten by the technical superiority of its adversaries, and obliged to throw division after division, unprotected, into the cauldron of annihilation." - Captain Von Henting

"Somme. The whole history of the world cannot contain a more ghastly word." -Friedrich Steinbrecher

An accurate, fair assessment is CERTAINLY possible at this time. /u/elos_ has given extensive answers on this, but allow me to lay out some of the basics.

Recommended Reading on The Somme

William Philpot's Bloody Victory (AKA Three Armies on the Somme) is the best history of the campaign I've read.

Robin Prior and Trevor Wilson's The Somme is also good, though their polemic-esque tone at times is off-putting.

Jack Sheldon's German Army on the Somme 1914-16, and Christopher Duffy's Through German Eyes: The British and the Somme are essential for the German perspective on the battle.

Elizabeth Greenhalgh's War by Coalition and Robert Doughty's Pyrrhic Victory both give French perspectives on the battle, and both are highly commendable as histories besides.

Chris McCarthy has a British history of the battle, examining ALL of the Battalion actions; incredibly in depth, hard to find and expensive, but certainly worth the money (I only wish I had a copy:( ).

Peter Hart has two books on the Somme, The Somme and Somme Success: The Royal Flying Corps over the Somme; both of are pretty good, though his narrative style may be hard to follow (LOTS of primary accounts).

For some accounts of those who were there:

• Ernst Junger's Storm of Steel
• David Jones' In Parenthesis
• Up To Mametz - and Beyond by Llewellyn Wyn Griffith
• 12 Days on the Somme by Sidney Rogerson
• Frederic Manning's Her Privates We (fiction, but HEAVILY based off his experiences on the Somme).

Wow, I feel sobered up. What an intensely sad, world changing war. Amazing write up by /u/duxbelisarius