The Battle of the Somme, also known as the Somme offensive, was a battle of the First World War fought by the armies of the British Empire and French Third Republic against the German Empire. It took place between 1 July and 18 November 1916 on both sides of the upper reaches of the Somme, a river in France. The battle was intended to hasten a victory for the Allies. More than three million men fought in the battle and one million men were wounded or killed, making it one of the deadliest battles in human history.

The French and British had committed themselves to an offensive on the Somme during the Chantilly Conference in December 1915. The Allies agreed upon a strategy of combined offensives against the Central Powers in 1916 by the French, Russian, British and Italian armies, with the Somme offensive as the Franco-British contribution. Initial plans called for the French army to undertake the main part of the Somme offensive, supported on the northern flank by the Fourth Army of the British Expeditionary Force (BEF). When the Imperial German Army began the Battle of Verdun on the Meuse on 21 February 1916, French commanders diverted many of the divisions intended for the Somme and the "supporting" attack by the British became the principal effort. The British troops on the Somme comprised a mixture of the remains of the pre-war army, the Territorial Force and Kitchener's Army, a force of wartime volunteers.

On the first day on the Somme (1 July) the German 2nd Army suffered a serious defeat opposite the French Sixth Army, from Foucaucourt-en-Santerre south of the Somme to Maricourt on the north bank and by the Fourth Army from Maricourt to the vicinity of the AlbertBapaume road. The 57,470 casualties suffered by the British, including 19,240 killed, were the worst in the history of the British Army. Most of the British casualties were suffered on the front between the AlbertBapaume road and Gommecourt to the north, which was the area where the principal German defensive effort (Schwerpunkt) was made. The battle became notable for the importance of air power and the first use of the tank in September but these were a product of new technology and exceedingly unreliable.

At the end of the battle, British and French forces had penetrated 6 mi (10 km) into German-occupied territory along the majority of the front, their largest territorial gain since the First Battle of the Marne in 1914. The operational objectives of the Anglo-French armies were unfulfilled, as they failed to capture Pronne and Bapaume, where the German armies maintained their positions over the winter. British attacks in the Ancre valley resumed in January 1917 and forced the Germans into local withdrawals to reserve lines in February before the scheduled retirement by about 25 mi (40 km) in Operation Alberich to the Siegfriedstellung (Hindenburg Line) in March 1917. Debate continues over the necessity, significance and effect of the battle.

The first day on the Somme, 1 July 1916, was the beginning of the Battle of Albert (1–13 July), the name given by the British to the first two weeks of the 141 days of the Battle of the Somme (1 July–18 November) in the First World War. Nine corps of the French Sixth Army and the British Fourth and Third armies attacked the German 2nd Army (General Fritz von Below) from Foucaucourt to the south of the Somme, northwards across the Somme and the Ancre to Serre and at Gommecourt, 2 mi (3 km) beyond, in the Third Army area. The objective of the attack was to capture the German first and second defensive positions from Serre south to the Albert–Bapaume road and the first position from the road south to Foucaucourt.

The German defence south of the road mostly collapsed and the French had "complete success" on both banks of the Somme, as did the British from Maricourt on the army boundary with the French northwards. XIII Corps took Montauban and reached all its objectives and XV Corps captured Mametz and isolated Fricourt. The III Corps attack on both sides of the Albert–Bapaume road was a disaster, making only a short advance south of La Boisselle, where the 34th Division suffered the most casualties of any Allied division on 1 July. Further north, X Corps captured the Leipzig Redoubt (an earthwork fortification), failed opposite Thiepval and had a great but temporary success on the left flank, where the German front line was overrun and Schwaben and Stuff redoubts captured by the 36th (Ulster) Division.

German counter-attacks during the afternoon recaptured most of the lost ground north of the Albert–Bapaume road and more British attacks against Thiepval were costly failures. On the north bank of the Ancre, the attack of VIII Corps was a disaster, with large numbers of British troops being shot down in no man's land. The VII Corps diversion at Gommecourt was also costly, with only a partial and temporary advance south of the village. The German defeats, from Foucaucourt to the Albert–Bapaume road, left the German defence on the south bank incapable of resisting another attack; a substantial German retreat began from the Flaucourt plateau to the west bank of the Somme close to Péronne. North of the Somme in the British area, Fricourt was abandoned by the Germans overnight.

Several truces were observed to recover wounded from no man's land on the British front; the Third Army diversion at Gommecourt cost 6,758 casualties against 1,212 German and the combined casualty count with the Fourth Army reached 57,470, (19,240 of which had been fatal). The French Sixth Army suffered 1,590 casualties and the German 2nd Army suffered 10,000–12,000 casualties. Orders were issued to the Anglo-French armies to continue the offensive on 2 July; a German counter-attack on the north bank of the Somme by the 12th Division, intended for the night of 1/2 July, took until dawn on 2 July to begin and was destroyed by the French and British troops opposite. Since 1 July 1916, the British casualties on the First Day and the "meagre gains" have been a source of grief and controversy in Britain.