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.
The immediate cause of World War I that made the aforementioned items come into play (alliances, imperialism, militarism, nationalism) was the assassination of Archduke Franz Ferdinand of Austria-Hungary. In June 1914, a Serbian-nationalist terrorist group called the Black Hand sent groups to assassinate the Archduke.
