Hopefully somebody out there may remember this, to me a very old book, i cannot remember the title or the author(I thought Banks but apparently not)
A man is surgically changed to look like a different species and dropped on 1 of their worlds as a guerrilla to prepare for his side to invade. At the end the world is taken but his side does not believe he is theirs and is in a concentration camp until the people who sent him come to look for him to make him do it again.