According to FirstWorldWar.com, World War I was triggered by the assassination of the Archduke Ferdinand of the Austro-Hungarian throne in June 1914 by a Serbian nationalist group. Although the group operated outside of Serbian governmental authority, the country was held responsible for starting the war. Austria-Hungary eventually responded by suppressing the nationalist movement within Serbia and increasing their presence in the Balkans, which resulted in conflict between Russia and Europe.
About.com explains that the conflict between Austria-Hungary eventually escalated into a full-scale war, causing Russia to get involved in defense of Serbia. Russia was threatened by the Austro-Hungarian invasion of the Balkans, and Germany took the opportunity to act on long-standing tensions to declare war on Russia as well. Bulgaria remained on the Austro-Hungarian side, while France, Japan, Britain, Italy and eventually the United States joined forces with the allies against Germany and Austria-Hungary. Even though the catalyst of the war was the assassination of the Archduke Ferdinand, historians agree that such a large-scale conflict had been coming for quite some time due to serious tensions between these heavily interconnected nations. First World War notes that this long-standing tension was termed the "Great War," even before the first battle began.