The Battle of Arras, also known as the Second Battle of Arras, was a British offensive on the Western Front during the First World War. From 9 April to 16 May 1917, British troops attacked German defences near the French city of Arras on the Western Front. The British achieved the longest advance since trench warfare had begun, surpassing the record set by the French Sixth Army on 1 July 1916 in the First day on the Somme. The British advance slowed in the next few days and the German defence recovered. The battle became a costly stalemate for both sides and by the end of the battle, the British Third Army and the First Army had suffered about 160,000 casualties and the German 6th Army about 125,000.

For much of the war, the opposing armies on the Western Front were at a stalemate, with a continuous line of trenches from the Belgian coast to the Swiss border. The Allied objective from early 1915 was to break through the German defences into the open ground beyond and engage the numerically inferior German Army (Westheer) in a war of movement. The British attack at Arras was part of the Anglo-French Nivelle Offensive, the main part of which was the Second Battle of the Aisne 50 mi (80 km) to the south. The aim of the French offensive was to break through the German defences in forty-eight hours. At Arras the Canadians were to capture Vimy Ridge, dominating the Douai Plain to the east, advance towards Cambrai and divert German reserves from the French front.

The British effort was an assault on a relatively broad front between Vimy in the north-west and Bullecourt to the south-east. After a long preparatory bombardment, the Canadian Corps of the First Army in the north fought the Battle of Vimy Ridge, capturing the ridge. The Third Army in the centre advanced astride the Scarpe River and in the south, the Fifth Army attacked the Hindenburg Line (Siegfriedstellung) but made few gains. The British armies then conducted smaller attacks to consolidate the new positions. Although these battles were generally successful in achieving limited aims, they came at considerable cost.

When the battle officially ended on 16 May, the British had made significant advances but had been unable to achieve a breakthrough. New tactics and the equipment to exploit them had been used, showing that the British had absorbed the lessons of the Battle of the Somme and could mount set-piece attacks against field fortifications. After the Second Battle of Bullecourt (3–17 May), the Arras sector became a quiet front, typical of most of the war in the west, except for attacks on the Hindenburg Line and around Lens, culminating in the Canadian Battle of Hill 70 (15–25 August).

At the beginning of 1917, the British and French were still searching for a way to achieve a strategic breakthrough on the Western Front. The previous year had been marked by the costly success of the Anglo-French offensive astride the River Somme, while the French had been unable to take the initiative because of intense German pressure at Verdun until after August 1916. The battles consumed enormous quantities of resources while achieving virtually no strategic gains on the battlefield. The cost to Germany of containing the Anglo-French attacks had been enormous and given that the material preponderance of the Entente and its allies could only be expected to increase in 1917, Field Marshal Paul von Hindenburg and General Erich Ludendorff decided on a defensive strategy on the Western Front for that year. This impasse reinforced the French and British commanders' belief that to end the stalemate they needed a breakthrough; while this desire may have been the main impetus behind the offensive, the timing and location were influenced by political and tactical considerations.

The mid-war years were momentous times. Governing politicians in Paris and London were under great pressure from the press, the people and their parliaments to win the war. Hundreds of thousands of casualties had been suffered at the battles of Gallipoli, the Somme and Verdun, with little prospect of victory in sight. The British Prime Minister, H. H. Asquith, resigned in early December 1916 and was succeeded by David Lloyd George. In France, Prime Minister Aristide Briand, along with Minister of Defence Hubert Lyautey were politically diminished and resigned in March 1917, following disagreements over the prospective Nivelle Offensive. The United States was close to declaring war on Germany; American public opinion was growing increasingly incensed by U-boat attacks upon civilian shipping, which had begun with the sinking of RMS Lusitania in 1915 and culminated in the torpedoing of seven American merchantmen in early 1917. The United States Congress declared war on Imperial Germany on 6 April 1917 but it would be more than a year before a suitable army could be raised, trained and transported to France.

It was agreed in the London Convention of 16 January 1917, that the French assault on the Aisne would begin in mid-April and that the British would make a diversionary attack in the Arras sector approximately one week prior. The French, Russians and British intended to launch a joint spring offensive in 1917 but this strategy foundered in February when the Russians announced that they could not meet their commitments. The spring offensive was reduced from attacks on the Eastern and Western fronts to a French assault along the Aisne River. In March, the German army in the west (Westheer), withdrew to the Hindenburg line in Operation Alberich, negating the tactical assumptions of the French plan. British and French troops followed up the German retreat into the old Noyon Salient up to the new German fortifications of the Hindenburg line (Siegfriedstellung) and French doubts over the wisdom of the offensive increased and the British were wary of proceeding, given the rapidly changing tactical situation. The new French commander-in-chief General Robert Nivelle, the victor of Verdun, claimed that he could combine overwhelming artillery-fire and skilful infantry tactics to break through the Western front in 48 hours. Lloyd George, the Prime Minister since 7 December 1916, accepted the claims of Nivelle, that if the British launched a diversionary assault to draw German troops away from the Aisne sector, the French offensive could succeed.

Three of the armies of the British Expeditionary Force (BEF, Field Marshal Sir Douglas Haig) were in the Arras sector, the Fifth Army (General Hubert Gough) in the south, the Third Army (General Edmund Allenby) in the centre and the First Army (General Henry Horne) in the north and the plan was devised by Allenby. The British used the lessons of the Somme and Verdun the previous year and planned to attack on an 11 mi (18 km) front, from Vimy Ridge in the north to Neuville-Vitasse, 4 mi (6.4 km) south of the Scarpe river. The preliminary bombardment was planned to last about a week except for a much longer and heavier barrage at Vimy Ridge.