"The Great War" or World War I was the result of nationalism supporting the aggression of the leaders of other countries in the European nation. Economic and imperial competition and fear of war prompted military alliances and an arms race, which further escalated the tension contributing to the outbreak of war.
The main causes of World War I were nationalism, imperialism, and military expansion. Nationalism was a major cause because it was ignored by many groups who felt that people of the same ethnic origin, language, and political ideals had the right to independent states. This caused conflict and left Germany and Italy divided because some groups felt strong about national autonomy which caused them to be subject to local dynasts or to other nations. Parts of Italy were left under foreign control. After the unification of Italy was accomplished in 1861, and that of Germany in 1871 there was still a problem with nationalism which was unresolved in other areas of Europe, resulting in tensions both within the regions involved and between various European nations. One particularly prominent nationalistic movement, Panslavism, figured heavily in the events preceding the war.
Another cause of "The Great War" was imperialism. Imperialism was a major cause because Great Britain, Germany and France needed foreign markets after the increase in manufacturing caused by the Industrial Revolution. The three countries competed for economic expansion over Africa. This caused plenty of clashing between France and Great Britain, and between Germany on one side and France and Great Britain on the other, almost precipitated a European war.
One of the most important causes of World War I was the military expansion. The tension caused by imperialism made each individual nation build up its military status. As a result of such tensions, between 1871 and 1914 the nations of Europe adopted domestic measures and foreign policies that in turn steadily increased the danger of war.