Final answer:
Militarism in the MAIN causes refers to the expansion of military forces and the arms race in Europe prior to World War I. This buildup of militaristic power led to heightened tensions and a propensity towards conflict, contributing significantly to the outbreak of war.
Step-by-step explanation:
The M in the MAIN causes stands for Militarism, which is the belief in building a strong military to prepare for war. Militarism contributed to the beginnings of World War I by creating an arms race that increased tensions among European nations. Countries expanded their armies and navies, which led not only to an atmosphere of distrust but also to a feeling of readiness for conflict, since nations were now more capable of waging war due to their significantly enhanced military capabilities.
The increasing militarization acted as a deterrent at first, but as armies grew, so did the concept of being too powerful to be defeated, which in turn made the idea of preemptive military action more appealing. The vast industrial capacities of countries like the United States, which joined the war later, also meant that they could produce supplies and weaponry at a pace that turned the tide in favor of the Allies. This industrial and military buildup ensured that when war broke out, it would be fought with a lethality and scale previously unknown, as seen with trench warfare and modern weapons technology.