World War 1 began in 1914 after the assassination of Archduke Franz Ferdinand of Austria-Hungary by a Serbian nationalist. This caused Austria-Hungary to declare war on Serbia, drawing in allies on both sides through a system of entangling alliances. Trench warfare ensued between the opposing armies in Europe, resulting in millions of casualties over the next several years as poison gas, tanks, airplanes, and submarines were used for the first time in modern warfare. The United States remained neutral at first, though it would eventually enter the war in 1917.