What year saw the largest increases of military personnel and what event brought about this increase?

The largest increases in military personnel occurred in 1940, driven primarily by the onset of World War II. As tensions escalated globally, many nations, including the United States, significantly expanded their armed forces through conscription and recruitment efforts to prepare for potential involvement in the conflict. This mobilization was crucial as countries sought to bolster their military capabilities in response to the growing threats posed by Axis powers.