I just finished a small book called "Sitting Ducks" by Steve Anderson. It's a study of Operation Greif, the Skorzeny infiltration of the American lines in the Ardennes by English speaking volunteers in GI uniforms and jeeps. There's some interesting info in the book and it's worth a read, but he attributes the genesis of the plan to Hitler himself, and says he got the idea from a similar American operation in Aachen where Americans used German uniforms and panzers to infiltrate into the city. I've never heard of such an American operation. Does anybody know where this story might have originated?