The war in 1915 was restricted to fewer longer skirmishes due to the entrenched troops. The increase of lethal firepower, both machine gun and field artillery, revolutionised combat tactics.
The advantage was now with the defence, being able to bring up resources to limit invasion before the enemy could move forward adequate reserves and artillery to exploit a breakthrough. The continuous battle line on the Western Front prevented classical offensive manoeuvres. The Germans, recognised this change long before the Allies, and adopted an flexible defence, in two widely separate lines, highly organised with entrenchments and barbed wire, heavy in machine guns, and supported by artillery. Assaulting troops broke through the first line only to be almost demolished by the fire from the succeeding line and pounded by artillery beyond the range of their own guns.
January 19-20: During the night: bombing attacks on Britain by Zeppelin dirigible airships under the control of the German navy resulted in few casualties, causing more anger than panic. During that year 18 more such raids occurred. The largest killed 59 people in London on 13 October.
January 31: The Central Powers, reinforcing their armies in the east, launched a great offensive under Hindenburg with the Battle of Bolimov, a bluff by the German Ninth Army, designed to distract Russian attention. Poison gas shells were used for the first time, but they were not highly effective in the freezing temperatures, and the Russians did not report the gas attack.
March 10: At the beginning of the year the Allies continued futile offensives in Artois and Champagne. The British failed at Neuve Chapelle, after nearly achieving a breakthrough. French casualties approached 400,000 during this period; British and German losses were also heavy.
April 22: Italy joined war against Austria. Allied preparations for another co-ordinated offensive at Ypres were spoiled by a surprise German attack preceded by a cloud of chlorine gas emitted from about 5,000 cylinders, the first time in history gas was used in this manner on a large scale. Two German corps drove through two intimidated French divisions and bit deeply into British lines, creating a wide gap. The Germans, however, had no reserves available, most of their troops having been diverted to the Eastern Front. Local counterattacks by the British finally cut off the German advance after bitter fighting.
April 25, 1915-January 8, 1916: Gallipoli Campaign.
At the instigation of Winston Churchill an unsuccessful major land and sea operation was attempted. The action was confined to the Dardanelles Strait and the tip of the Gallipoli Peninsula near Istanbul The campaign was fought mainly by Australian and New Zealand (ANZAC) forces, who suffered heavy losses in an attempt to force their way through the Dardanelles and link up with Russia. Initially it was to have been exclusively a naval operation. However this failed in February 1915 when several British and French ships were damaged by floating mines.
April 26: Britain, France, Russia, and Italy secretly signed the Treaty of London. It promised Italy territorial gains (at the expense of AustriaHungary) providing that it entered World War I on the side of the Triple Entente (Britain, France, and Russia). Italy's intervention did not achieve the rapid victories expected, and the terms of the treaty (leaked by Russia in 1918) angered the USA. Britain and France refused to honour the treaty and, in the post-war peace treaties, Italy received far less territory than promised.
May - June: A combined attack by the British and French along the front between Neuve Chapelle and Arras advanced troops 2½ miles into the German trench system, but they failed to break through
May 7: German submarine sank ocean liner Lusitania, later helping to bring USA into the war.
May 23: Baited by shrewd Allied diplomacy offering vast territorial gains, Italy declared war on Austria. The Italian army was about 875,000 strong, but it was deficient in artillery, transport, and ammunition reserves.
September-October: The Allies again launched unsuccessful offensives: the Second Battle of Champagne and the Third Battle of Artois. The minor gains made were out of proportion to the casualties suffered: more than 200,000 French, nearly 100,000 British, and 140,000 Germans.
October 6: In anticipation of the Bulgarian declaration of war, a strong Austro-German drive, was launched from Austria-Hungary into Serbia. By the end of 1915 the Central Powers had conquered all of Serbia and eliminated the Serbian army as a fighting force.
October 14: After Bulgaria declared war on Serbia the Allied troops advanced into Serbia.