Nationalism: feeling of patriotic loyalty. This unified the countries of 2 very active players in World War 1- Italy and Germany. Nationalism also motivated minorities to break-off and fight for their own countries.
Imperialism: desire to gain control over territory outside of one's own country
I want to gain land that is in a different country than mine such as Africa or Asia!
Militarism: the massive buildup of military
Alliances: countries that have alliances with each other will fight with each other, turning a war that could have stayed between 2 countries and turning it into a bloody battle.
The Fuse: The Assassination of Archduke Franz Ferdinand in Sarajevo. He was assassinated by Gabriel Princips, a Serbian nationalist. The Archduke was in Sarajevo to inspect the military.
The sinking of the Lusitania. This event triggered many Americans, making the will to go to war easy.