Think of the acronym MAIN. M stands for militarism. Britain had the strongest navy during that time, and Germany threatened to pass Britain based on military strength thus resulting in an arms race between the two nations. A stands for alliances. Germany, Italy, and Austria-Hungary formed the Triple Alliance, and the Triple Entente was formed by Britain, France, and Russia. Imperialism is another cause behind the war. European powers such as Germany and France scrambled for colonies in Africa, Asia, and the Pacific. Nationalism is also one of the major causes of the war. Though it may unite a country, it also can lead to nation against nation. An example of this is Serbia, which wanted to nationalize with Austria-Hungary, but that did not work, and this triggered the assassination of Archduke Franz Ferdinand, the heir to the Austro-Hungarian throne and his wife.
