World War I alliances

The two opposing groups that fought in World War I were the Central Powers (Germany, Austria-Hungary, Bulgaria, and Turkey) and the Allies, an alliance that grew from the three members of the Triple Entente (Britain and the British Empire, France, and Russia) to incorporate 27 Allied and Associated powers, including Italy and, towards the end of the war, the USA. Hostilities were precipitated by the murder of Archduke Franz Ferdinand, heir to the Austro-Hungarian empire, by a Serbian nationalist. The incident led to unreasonable demands being made upon Serbia by Austria-Hungary and the eventual outbreak of war.

The alliance systems that were brought into play during World War I had been built up since 1882, when the Triple Alliance was formed between Germany, Austria-Hungary, and Italy to counter the power of Russia and France. In 1904 Britain and France signed the Entente Cordiale, an agreement designed to check German colonial ambitions. The Entente Cordiale formed the basis of the Triple Entente made between Britain, France, and Russia in 1907; the Triple Entente became a military alliance in 1911. Although the Triple Alliance was reaffirmed in 1912, Italy did not join the Central Powers at the outbreak of war in 1914, and later joined the Allies.

In addition to the powers involved in the Triple Alliance and Triple Entente, many other nations joined the war after 1914. Romania, Greece, Belgium, Serbia, Montenegro, Portugal, Italy, and the USA fought alongside the original members of the Triple Entente, forming the Allies. Turkey and Bulgaria joined the remaining members of the Triple Alliance to form the Central Powers.