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 River Somme in France.

The battle was intended to hasten a victory for the Allies and was the largest battle of the war's Western Front.

More than three million men fought in the battle and one million men were wounded or killed, making it one of the bloodiest battles in human history.

The French and British had committed themselves to an offensive on the Somme during Allied discussions at Chantilly, Oise, 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 regular army; the Territorial Force; and Kitchener's Army, a force of volunteer recruits including many Pals' Battalions, recruited from the same places and occupations.

The first day on the Somme (1 July) saw a serious defeat for the German Second Army, which was forced out of its first position by 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 Albert–Bapaume road.

The first day on the Somme was, in terms of casualties, also the worst day in the history of the British Army, which suffered 57,470 casualties, including 19,240 killed in action.

These occurred mainly on the front between the Albert–Bapaume road and Gommecourt, where the attack was defeated and few British troops reached the German front line. The battle became notable for the importance of air power and the first use of the tank in September.

Tanks were still in the stages of development and were prone to breaking down.

At the end of the battle, British and French forces had penetrated 10 km (6 mi) into German-occupied territory.

This was their largest territorial gain since the Battle of the Marne in 1914.

However, key objectives of the Anglo-French armies were unfulfilled, as they failed to capture Péronne and halted 5 km (3 mi) from 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 to the Siegfriedstellung (Hindenburg Line) began in March. Debate continues over the necessity, significance and effect of the battle.