The early twentieth-century was full of political tension. Europe's great powers were progressively coming to standoffs over acquiring new colonies. At the same time the Ottoman Empire was slowly crumbling. Bulgaria, Greece, Romania, and Serbia became independent affecting the balance of power in Europe. However open hostilities were triggered by the assassination of the heir to the Austro-Hungarian throne, Archduke Francis Ferdinand by a Serbian nationalist in 1914. Austrians seeking vengeance blamed the Kingdom of Serbia and attacked Belgrade. The Serbs were allied with Russia and asked for their assistance. Germany alarmed by Russian mobilization called for them to stand down. Austria declared war on Serbia, their Russian allies declared war on Austria. Germany allied with Austria declared war on Russia. Russian allies France and Britain declared war on Germany and World War I began.
The combatants were aligned in two opposing alliance; the Allies consisting of United Kingdom, France, and Russia and the Central Powers consisting of Germany and Austria. Italy joined the Central Powers after the Treaty of London in which Italy was to gain territory in Austria and Turkey. Early in the war the Central Powers achieved some victories as Russia was ill-equipped for the war and Britain and France were unable to coordinate their efforts to fight.
From the beginning of the war President Woodrow Wilson maintained a policy of strict neutrality. The only concession to this was to provide material assistance to the Allies. The United States remained neutral following the May 1915 sinking of the British ocean liner Lusitania killing 128 citizens of the United States, although the American public was in an uproar. In 1916 President Wilson was re-elected having campaigned on an antiwar and pro-neutrality platform, the war in Europe had left millions dead with neither side poised to achieve victory.