A general response would be M.A.I.N. (Militarism, Alliances, Imperialism, and Nationalism). As each major European power had practice such ideas and ultimately lead them to into battle with each other.
However to put (M.A.I.N) in context, upon the assassination of Archduke <em>Archduke Franz Ferdinand</em> by Serbian separatist, which ultimately lead to the Austria-Hungarian Empire declaring war against Serbia. However, Serbia had an alliance with Britain, France, and Russia (Allies). Whilst, Austria-Hungary had alliances with the Ottoman Empire & Germany (Central Powers). Which subsequently enacted every major European power to declare war on the basis of their alliance's agreement.