Operation Ironside formed part of Operation Bodyguard, a broad strategic military deception intended to confuse the Axis high command as to Allied intentions during the lead-up to the Normandy landings. The overall aim of Bodyguard was to tie down German forces away from Normandy by threatening other targets. Ironside's specific objective was to tie up the 17th SS and 11th Panzer divisions deployed in the south of France.
Overall planning for Bodyguard and Ironside rested with John Bevan and the London Controlling Section (LCS). The LCS had been set up in 1942 following successes in deception in the Middle East by Dudley Clarke. After initial attempts at deception planning the department was tasked with bringing Bodyguard to fruition. One of their most useful deception channels was through double agents. During the early stages of the war, the Abwehr (German intelligence) had sent spies to Britain, but all of them either surrendered or were captured. Some, along with other volunteers, were used as an extensive misinformation network under the control of the Twenty Committee.
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.
According to the storyline for Ironside, ten days following D-Day, Allied forces would land in the Bordeaux region. This force would spend around twelve days establishing a bridgehead before advancing to meet formations supposedly part of an invasion of the Mediterranean coast of France (in reality these were fictional landings as well, part of another Bodyguard deception called Operation Vendetta). The supposed target of Ironside was the mouth of the Gironde estuary, with a landing site at Royan.
At first, Bevan suggested that the fictional invasion force should stage from the American East Coast. Newman Smith, based out of New York and responsible for the US elements of the deception, felt this was an unrealistic story and suggested a large force from the US might conceivably reinforce a bridgehead established by units from the UK. Formations intended for Normandy could be "re-purposed" for the initial invasion. The final plan earmarked two Overlord divisions for the assault with the supposed reinforcements consisting of six real divisions (the 26th, 94th, 95th, and 104th Infantry, and the 10th and 11th Armored) under the notional command of Lieutenant General Lloyd Fredendall.