World War I began in 1914 due to increasing militarism, alliances, imperialism, and nationalism in Europe. By 1914, Europe was divided into two main alliance systems: the Triple Entente of Britain, France, and Russia, and the Triple Alliance of Germany, Austria-Hungary, and Italy. Tensions rose due to imperial competition and disputes over territory. The assassination of Archduke Franz Ferdinand by a Serbian nationalist sparked Austria-Hungary to declare war on Serbia, pulling the other European powers into war through their alliance obligations. Over the next four years, over 37 million casualties resulted from the massive industrialized warfare between the European powers.