First is the introduction of Imperialism. Major European powers became industrialized and imperialistic. The powers always competed to control overseas market and for colonies. Therefore, conflict and suspicion between the powers developed. Undoubtedly economic rivalries played a considerable part in creating international tensions before the WW1. However, the economic rivalries have been much overstated. Thus, the economic rivalries only played a minor part in causing the WW1. The second one was the Nationalism. There were two kinds of nationalism in 19th Century Europe. One was the desire of subject peoples for independence. It led to a series of national struggles for independence among the Balkan peoples. Other powers got involved and caused much instability. The other one was the desire of independent nations for governance and kudos. As the powers try to control each other in Europe, their rivalries were regarded as one of the causes of WW1. The third one was the Alliance System. It was started by Bismarck, the German Chancellor from 1871 to 1890. After the Franco-Prussian War, Bismarck held that Germany was a satiated state which should give up ideas of further conquest. Thus Bismarck organized a system of alliances designed to maintain Germany's hegemony on the European continent. Bismarck tried to befriend Austria, Russia, Italy and Britain in order to isolate France. The alliances were made in secret and so produced much distrust and suspicion among the European powers. Since the European powers had made alliances with one another, local war might develop into a world war easily. The last one was Militarism. Militarism denoted a rise in military expenditure, an increase in military and naval forces and a preference for force as a solution to problems. After 1871, the war atmosphere engendered by the secret alliances led to an armament race among the powers.
I think that Militarism was the main and the most important cause of WW1 as the increased military and naval rivalry led not only to the belief that war was coming.