The alliances that formed between six major European countries prior to 1914 were certainly one of the major causes of World War I. Because the two alliances that formed before the war were fairly evenly balanced, the war dragged on for years.
The web of treaties and alliances formed in Europe before the war made conflict all but inevitable. These alliances were basically promises by nations to come to the aid of their allies in case they were attacked. Anything was likely to set off such a powder keg of possibilities, and the spark that lit the fuse was the 1914 assassination of Austrian Archduke Franz Ferdinand. By this time, Germany, Austria-Hungary, and Italy had formed the Triple Alliance, also known as the Central Powers, and the United Kingdom, France, and the Russian Empire had formed the Triple Entente (Allied Powers). Within weeks after the assassination, these two alliances were in all-out war.
Countries that sided with one alliance or the other were known as associated powers. The associated powers aligned with the Triple Alliance included Turkey and Bulgaria. Countries that aligned with the Triple Entente included Japan, Belgium, Greece, Serbia, Rumania, and Montenegro. In 1917 the Russians withdrew from the war effort, but in April 1917, the United States declared war on Germany and entered the conflict. This changed the course of the war and eventually enabled the Allied Powers to emerge victorious.