War exploded across Europe in August 1914 as the result of key decisions taken by European leaders who tried to use the assassination of Archduke Franz Ferdinand in Sarajevo as a pretext for securing greater territorial, military, and economic strength at the expense of their rivals. With the Central Powers of Germany, Austria-Hungary, and the Ottoman Empire squaring off against more than 20 Entente nations (also known as the Allies) led by Britain, France, and Russia, U.S. president Woodrow Wilson made a decision to stay neutral. Neutrality proved difficult to maintain, and for 2½ years the United States found itself embroiled in a series of diplomatic crises that gradually edged the nation nearer to war.