Training accommodation for more than 170 officers and 2,600 men of the United States Navy made Falmouth Sub-Command an important base for the D-Day assault. Loaded with personnel, vehicles and equipment, 38 L.S.T.s sailed to join the D-Day armada, and an average of four M.T. Ships a day ferried vital transport and supplies in the invasion build-up.
Falmouth was commissioned on Sept. 3, 1939, as H.M.S. Forte. The Imperial Hotel became H.M.S. Forte II. In December 1939 a Contraband Control Base was opened. Defence Booms were laid in the harbour, St. Mawes Bay, Falmouth Bay and the Helford River, the last being removed in May 1945. First test of Falmouth's improvisation came late one night in May 1940, when ships of the Royal Netherlands Navy arrived after escaping from Holland.
Hundreds of troops passed through Falmouth on their way to France in early June 1940. After Dunkirk, thousands of troops and refugees, brought over in 243 ships, were cared for by W.R.N.S. and W.V.S. personnel and housed in cinemas and the Princess Pavilion. June also brought the first air raids against the Dockyard. In eight raids during the month one ship was sunk, two set on fire, and considerable damage was done to the Northern Arm of the Dockyard.
An Auxiliary Patrol was formed, consisting of yachts, trawlers, drifters and motor boats, guarding the harbour and the boom entrance until finally paid off in June 1945. Observation minefields, a balloon barrage and anti-submarine fixed defences also protected the harbour, and in May 1941 a Coastal Force base was set up at Coastlines Wharf. After D-Day, Coastal Force activity at Falmouth diminished, and in October 1944 the base closed down. Air raids and harbour mining activity caused several casualties during 1941.
The King and Queen visited Falmouth in May 1942, when it was decided that the port should be used as a Combined Operations Base. In July the following year it was decided that the United States Navy should take over the organization. The first U.S. contingent arrived at St. Mawes on August 18, 1943, establishing a U.S.N. Advanced Amphibious Training Sub Base in requisitioned buildings; mooring buoys were laid and five loading hards built before D-Day. The base closed in 1944.