World War I was primarily caused by the Russian Revolution. European countries were afraid that the communist ideals of the Bolsheviks would flow to the west and effectively toppled down their empires. As war started to look more and more likely, France sent half a million troops to reinforce Quebec-Louisiana from Mexico and the USA (both of whom would´ve entered the war against France because of treaties). This was frowned upon by Mexico and the USA, who bordered Louisiana. Both these countries massed their forces at the border and prepared for war in a manner similar to European countries.