The Great War of 1914 also known as World War 1 was one of the bloodiest wars in history that killed millions of people and happened for many reasons, some of the reasons haven't even been found. The spark that triggered the war was the assassination of the Archduke Ferdinand of Austria-Hungary and his wife Sophie on June 24, 1914 by the hands of Gavrilo Princip, a member of the Black Hand in Serbia (Background Essay), but that wasn't one of the deeper causes of WWI. There were four underlying causes of WWI; the Alliance System, Nationalism, Imperialism, and Militarism. Out of the four, I think the most important cause of World War I was Militarism.
The first underlying cause for WWI was the alliance system among the European countries. There are countries in Europe that were evolving at a faster rate than other countries. In order to keep peace, the United Kingdom, Russia, and Prance agreed to be alliances and Germany, Austria-Hungary, and Italy become alliances as well. The United Kingdom, Russia, and France called their alliance Triple Entente and Germany, Austria-Hungary, and Italy called their alliance Triple Alliance (Document 3). The nations in the alliance system agreed to assist each other in time of need. When Austria-Hungary declared war on Serbia, Austria-Hungary had the back up of Germany while Serbia had the back up of Russia which in turn means back up of France and Britain as well. In the end, everyone was blaming each other for the cause of WWI, but if there wasn't an alliance system then there wouldn't be a World War in the first place (Document 4). The alliance system within the European nation was one of the causes for WWI.
The second underlying cause for World War I was Nationalism: the pride and devotion to one's nation. Gavrilo Princip was a nationalist who fought for his people's honor and freedom. The Serbians as a nation united together to fight for their freedom from Austria-Hungary in order to keep their identity and tradition (Doc 6).