During World War I, the term "mobilization" referred to getting troops and supplies together and ready for war. Russia first used this term in a military context starting in the 1850s when they referred to getting their army ready.
In 1914, a country's enemies saw military mobilization as an act of war. The intricate mobilization plans played a role in the start of World War I due to this general custom and law of warfare. During the period leading up to World War I, the Great Powers did not have enough men on active duty to win a war. When it was time for war, they mobilized men for compulsory military service to obtain this manpower.