Final answer:
The "m" in the "'main' causes of World War I" represents Militarism.
Step-by-step explanation:
The "m" in the "'main' causes of World War I" represents Militarism. Militarism refers to the glorification of the military and a readiness for war. It was one of the long-term causes of World War I, along with Alliances, Imperialism, and Nationalism, which are represented by the other letters of the acronym MAIN. European nations engaged in an arms race, building up stores of weapons and military personnel. Standing armies and naval forces increased in size and readiness for mobilization.