The CDC was established on 17 March 1941, responsible for coordinating the defense of the Midwest region of the United States.

During the waterway's usable season from March through November, 90 percent of the United States' iron ore production for domestic use passed through it in 1939.

After Pearl Harbor in December 1941, fear of possible air or paratroop attacks by German forces led to a major expansion of defense measures.

Scenarios envisioned included U-boats in Hudson Bay launching attack aircraft, and one-way bombing or paratroop missions along a great circle route from German-occupied Norway.

By late 1943, with no threat emerging and spare components stockpiled in the event of lock damage, the US forces were cut to 2,500 troops, and the AA and air warning defenses were abandoned.