A fictitious army, First US Army Group (FUSAG) was formed with Patton in command. They were located in SE England, across from the Pas de Calais. There were rubber tanks and trucks and lots of fake radio traffic. It was all designed to make the Germans think Calais would be the target and not Normandy (or any of the other half dozen possibilities). They believed Patton would lead the invasion, as he had the American invasion of Sicily. Even better, they knew as a historian Patton would advocate for a landing at Calais (which at some point he did).
Patton knew from the beginning of 1944 that he would be given command of Third Army and lead the Normandy breakout when it happened, but that was another secret that never got out.
The operation succeeded brilliantly. The German 15th Army was still sitting in far northern France when Patton began his dash to the German border. They never saw action in Normandy.
Ike's discipline of Patton is often misinterpreted. Ike wanted and needed Patton in France because he knew Patton was his best field army commander. His letter of reprimand and orders for an apology tour were intended to preempt harsher punishment from Washington, which probably would have included orders back to the States and no further role as a combat commander. At the end of the day, Marshall accepted Ike's actions as adequate and resisted calls from politicians for harsher punishment. Of course, the Germans thought the entire affair was ridiculous.
Similarly, an officer, can’t think of his name, was at a meeting in Europe where a low level bombing operation was being planned. He spoke up and said it was suicide. When the Colonel pushed back, the officer said he’d be glad to lead the mission if the Colonel sat in the plane with him.
The mission was scrubbed, but the Colonel was getting ready to hammer down on the officer. A high official got wind of what was happening and transferred the officer back to the States before the Colonel could act.
The officer was assigned the task of founding and training what would become the Navy’s UDT, later the SEALS. providing invaluable service in Tarawa and beyond.