This film is set just as the D-Day landings are being planned. A soldier wakes up in what he thinks is an American camp but it is a German camp completely disguised to make him think that he has woken up and missed the D-Day assault. In fact the germans are trying to cleverly get the date out of him so that they know exactly where to deploy the German troops. He only realises it is a trick when he noticed a cut on his hand which he did that morning and it hasn't healed. Any chance of the title. I seem to think it might be Operation... something?

36 Hours (1965)

"36 hours" is the film, can't remember the stars but very good concept. It may come up if you search on line.