In what ways was WWI caused by aggressive imperialism and warring military alliances?

Military alliances and imperialism are two of the four MAIN causes of WWI. MAIN stands for: Militarism, Alliances, Imperialism and Nationalism. 
It can be argued that alliances were the biggest contributor to the war. The alliances of the Allied and Central Powers meant that if one country went to war, all the other countries would follow. These alliances created a snowball effect that caused what would have been a smaller war to become a World War. As it has been described, the 'powder keg' was sitting and waiting to go off at any minute. The spark that lit the keg was the assassination of Archduke Ferdinand.
Imperialism was also a key contributor to the war. Imperialism led to the scramble for Africa, in which European countries rushed to fight over land. This led to increased tensions among the countries. Militarism increased due to the aggressive imperialism in order to protect the colonies and trade. This led to alliances as well in order for countries to protect their land and themselves.

WWI was largely driven through military alliances. Before WWI, there had been wars in the Balkans for generations. The region had a legacy of assassination attempts and bloody palace coups. The difference at the onset of WWI was that Serbia appealed to its Slavic neighbor and ally Russia for assistance when Austria-Hungary delivered an impossible ultimatum due to the assassination of Franz Ferdinand. When Russia mobilized against Austria-Hungary, it led to German mobilization. France, fearing attack and wanting to help out its ally Russia, mobilized against Germany. Without the alliance system, the war would have probably been a localized war between Serbia and Austria-Hungary. With the alliances, however, the war engulfed the entire continent and all of the colonies that the combatants possessed. It would even reach the United States.  
Aggressive imperialism helped to drive the war as well. Germany, late on the colonial scene, wanted to acquire colonies for raw materials and to allow Germans to migrate to other places and still be part of the German Empire. Germany developed a navy to protect its commercial and territorial interests overseas. Britain had a naval program which dictated that its navy must be the strongest in Europe, so the buildup of the German battleship fleet created a situation where Britain had to build theirs as well. This created an arms race that would eventually lead to war.