Militarism
Militarism is a belief or system where the military is exalted and its needs and considerations are given excessive importance or priority. As the factory system grew nations began to mass produce weapons such as machine guns, tanks, submarines, and even barb wire. Militarism was a powerful force at the turn of the century in Europe. While militarism alone did not start World War I, it fueled a potent arms race and undermined international diplomacy as a means of resolving disputes.

Alliances
Countries throughout the world have always made mutual defense agreements, or alliances, with their neighbors, treaties that could pull them into battle. These treaties meant that if one country was attacked, the allied countries were bound to defend them. Before World War I began, the following alliances existed: Russia and Serbia, Germany and Austria-Hungary, France and Russia, Britain & France & Belgium,  and Japan & Britain. Once the Archduke was killed, it was just a matter of time before nations began to mobilize for war, as their treaties expected them to do.

Imperialism
Imperialism was on the steady rise by 1914. Most of Africa was colonized and Asia too, with several European countries making competing in those locations. This led to several points of contention. Because of the raw materials these areas could provide, tensions around which nation had the right to exploit these areas ran high. For example, the ‘Scramble for Africa’ fueled rivalry and led to small incidents. However, the increasing competition and desire for greater empires led to an increase in confrontation that helped push the world into World War I.

Nationalism
Nationalism was a prominent force in early 20th century Europe and a significant cause of World War I. Nationalism is an intense form of patriotism or loyalty to one’s country. Nationalists exaggerate the importance or virtues of their home nation, placing its interests above those of other nations. This attitude, or pride, will absolutely lead to WWI when one nation will physically challenge another, specifically after the assassination of Archduke Franz Ferdinand as the Balkan region of Europe had great problems with nationalistic feelings. 

Assassination
The most immediate cause of WWI, and most obvious, was the Assassination of Archduke Franz Ferdinand of Austria-Hungary (the prince). In June 1914, a Serbian-nationalist terrorist group called the Black Hand sent a small group to assassinate the Archduke. Gavrilo Princip shot the Archduke and his wife while they were driving through Sarajevo, Bosnia which, at the time, was part of Austria-Hungary and they died of their wounds. The assassination of Ferdinand led to Austria-Hungary declaring war on Serbia. When Russia began to mobilize to defend its alliance with Serbia, Germany declared war on Russia. Thus began the expansion of the war to include all those involved in the mutual defense alliances.