Groundwork Laid for the Battle of the Somme

On December 6-8, 1915, the Allies met in France for the Second Chantilly Conference, which would lay the groundwork for World War I's Battle of the Somme, a four and a half month-long battle in France that would prove to be one of the war's bloodiest.

At the Second Chantilly Conference, held in early December 1915, the Allies agreed to coordinate simultaneous offensives to exhaust German resources and manpower. As part of this, the British and French agreed to a joint French-led offensive on the Somme River for the summer of 1916. But the Germans attacked the French at Verdun in February, forcing the British to shoulder the bulk of the planned Somme offensive, which developed the subsidiary purpose of relieving pressure on the French at Verdun.

The Somme offensive, stretching along a front 25 miles long, began with artillery barrages on June 24h that lasted a week. The plan was to so overwhelm the Germans with the bombardment that the infantry would have a relatively easy time. However, the bombardment was largely ineffective, which meant that when the infantry climbed out of the trenches on July 1 and crossed into No Man's Land, they were cut down by German machine guns and artillery. It was the single bloodiest day in British army history, with nearly 60,000 British casualties, a third of them killed.

While there was some success in breaking through the German front line along the southern part of the front on that first day of the battle, there was no real progress along the majority of the line. The Battle of the Somme would last for 4 1/2 months, with periods of renewed fighting. One of the most notable of these was the Battle of Flers-Courcelette, the first time tanks were used in battle.
By the time the Battle of the Somme finally ended in November with inconclusive results, both sides had sustained high casualties, with more than a million total killed, wounded, captured, or missing, making it one of the bloodiest battles of the war.