Bordeaux was an important port for the German war effort, receiving a great deal of cargo, mostly raw materials, from overseas. It was also a major naval base, with huge submarine pens for U-boats. The Gironde estuary and Bordeaux had already been a target for the Allies. Operation Frankton was a 1942 commando raid targeting important shipping in the port. In January 1944, the Allies intercepted communications indicating that German commanders were concerned by the possibility of landings in the Bay of Biscay region of France. The next month, German naval and air units undertook anti-invasion exercises in the area. Ironside was intended to amplify these concerns.