The Largest Factor Contributing to World War I.
One can examine the multitude of reasons for the outbreak of war in Europe by targeting the volatility of hostilities at the outbreak of the war. Alliances existing before the war may have been the most influential factor in triggering a "World War". There are other reasons why a war was inevitable such as Imperialism, Nationalism, the Arms Race, the crises in Bosnia and Morocco, and the assassination of the Archduke Ferdinand, heir to the Austria-Hungarian throne. I do believe, however, that when one examines the boiling point it becomes obvious that without the existing political alliances present throughout Europe at the time, such hostilities and military testiness couldn't have possibly erupted on such a large scale. .
Nationalism was an understandable effect of the existing situations. Italy and Germany had been unified not many decades earlier, Britain was enjoying the beneficial effects of an Industrial Revolution, and France was angry and thirsty for revenge over the loss of Alsace-Lorraine. Austria-Hungary and the Balkans had tensions throughout the many cultural groups comprising them. .
Imperialism was also a definite contributor with a deliberate and competitive attempt by western European to colonize Africa. The Moroccan crisis between France and Germany proved to be one "testing ground" for French allies Britain and Russia. While this "test" implemented by Germany never materialized into an actual battle, it was a telling and tense situation while it existed, and Germany was able to then gauge these powerful allies" abilities and shortcomings and preparedness for conflict. .
The Arms Race and plans for war such as the von Schlieffen plan also forced the European countries to reevaluate their ability to repel invasion and defend themselves. It became imperative to be able to not only assemble a vast army, but also be able to mobilize it quickly, and equip it with the most modern technological advances in weaponry.