World War I was caused by a variety of things. Militarism and war fever were big: the European nations were growing ever more powerful, and the armed forces of many of these countries i.e. Britain, Russia, and Germany getting stronger. Another cause was the alliance system conjured up by the German Otto von Bismarck. The alliance system entangled the involved countries with treaties and agreements that often contradicted each other, causing conflict between a few countries to escalate into a World War. Imperialism also posed a problem, as the most powerful nations in the world were scrambling to colonize and militarize then-under-developed nations in Africa, South America, Asia, and the Pacific. With nearly all of the great military powers rubbing shoulders to build up their own empires, a confrontation was inevitable. .
Yet another cause for World War I was the assassination of the Austro-Hungarian archduke Franz Ferdinand. The archduke was killed while in Sarajevo, and the killer was from a secret organization known as the "Black Hand". Austrian officials put the blame on Serbia for the death of their archduke, and eventually declared war. Because of the alliance system discussed above, the other European nations were roped into the conflict, causing it to become a World War. .
Mobilization of armed forces was another more immediate cause of World War I. Russia, in defense of Serbia, mobilized its troops on the border of Germany and Austria-Hungary. (Germany was an ally of Austria-Hungary.) Germany was already plotting to invade Russia through its Schlieffen Plan, which would require the German army to march through Belgium to invade France, take Paris, and then swing around and head for Russia. This Schlieffen Plan was supposed to prevent a war in which Germany would have an enemy on both sides by disabling the French. The Schlieffen Plan obviously failed. .
The results of the Great War were almost as numerous as the causes.