- 1 January
- Battle of Artois and Champagne in France begins.
- 31 January 1915
- The German Army makes use of poison gas for the first time in an attack on the Russian position west of Warsaw, Poland. The 18 000 gas shells are rendered ineffective as a result of the subzero temperatures.
- 3 February
- The Turkish army launches an unsuccessful attack against British controlled Suez Canal; used to ferry troops from India, Australia and New Zealand to the Western Front.
- 18 February
- Germany begins a U-Boat campaign, attacking merchant and passenger shipping around the British Isles.
- 10–13 March 1915
- Battle of Neuve Chapelle”” British and Germans suffer heavy losses. After three days of fighting the British suffer 11 000 casualties and the Germans 10 000 casualties.
- 22 April
- The Armenian Genocide begins: 250 Armenian intellectuals are rounded up and arrested in Turkey.
- 22 May
- Forced deportations of Armenians for the next two years lead to the deaths of 1.5 million Armenians due to starvation, thirst and murder by Turkish troops
- 22 April–22 May
- The Second Battle of Ypres begins””the first use of poison gas by the German Army on the Western Front resulting in 58 000 British casualties and 38 000 German casualties.
- 25 April
- 70 000 Allied troops, including 15 000 Australians and New Zealanders, land at Gallipoli Peninsula near Constantinople. The aim is to unblock the Dardanelles Straits, between Gallipoli and Constantinople, to gain access to Russia through the Black Sea. The Gallipoli Campaign ends in failure as the Turks succeed in defending the peninsula.
- 7 May
- Germans sink the British liner Lusitania.
- 4 August
- Germans capture Warsaw.
- 25 September 1915
- Battle of Artois between French and German Armies and the Battle of Loos between the British and German armies takes place. The British use poison gas for the first time. They succeed in capturing Loos, but the Germans regroup and when the British relaunch their offensive, they are decimated by German machine gunners. The British suffer a heavy casualty of 50 000 men.
- 3 October
- Anglo-French army lands at Salonika, Greece.
- 19 December
- Months of fighting between Turkish and Allied troops leads to a deadlock. The Turks successfully prevented a breakthrough, inflicting 250 000 casualties on the British forces. The British Navy begins the evacuation of 83 000 soldiers from the Gallipoli Peninsula.
- Sir Douglas Haig replaces Sir John French as commander of the British expeditionary Force in Europe.