At the start of the war France, Britain and Russia formed the Triple Entente, extending the existing Entente Cordiale between Britain and France. The aim was to curb the ambitions of the German Empire under Wilhelm II to become a major power. Italy joined the war in 1915 on the side of the Entente. On the other side were the Central Powers consisting of the German Empire, Austria-Hungary, Bulgaria and the Ottoman Empire. In 1917 the USA entered the war on the side of the Entente, marking a decisive turning point that was to lead to the military collapse of the Central Powers.
In many respects the First World War triggered a lasting shift in the power structure. For Germany and the Austro-Hungarian successor states, defeat meant the end of the old ruling systems. The October Revolution in 1917 not only caused Russia to withdraw early from the war but also marked the start of Communist Soviet Union. Through its participation in the war on the side of the victorious powers, the USA became a world power and has remained so to this day.