The Dirty Dozen was a group working for the OSS.
It is important to note that while the 'group' is part of the Office of Strategic Services during World War II, only a few of its members are actually affiliated with that organization, usually the officer leader of the ad hoc group and an NCO member of the MPs to act as lead guard.
Guard being the operative word here because the remaining members of the group(s) are all servicemen in military prison awaiting either execution or lengthy time behind bars. A couple were people in the wrong place at the very wrong time while a couple were basically good people pushed into doing very bad things. The remainder, however, were in their predicament for the excellent reason that they belonged there. Murderers, thieves, rapists, you name it. More than one is a certifiable nutcase who should never have been allowed outside again but since the missions that these men would be trained for and sent on were almost by definition suicide runs, if they did not make it, so be it.
But for those who might actually survive to come back alive, the promise of a pardon was given.
The first band of men that we meet are led by Captain (or Major, depending on the media) John Reisman, an Army officer who admittedly has a problem with some authorities above him but who is very much dedicated to getting the job done and will willingly give his all doing so. We will have the opportunity of following him in three missions (one told as a novel and a film, albeit with differences, one more as a novel alone, and one more just as a film). Reisman is about as rugged as they come and when those being freed from their cells get the idea that they can run roughshod over him or bamboozle him or just annoy him to compliance, he very quickly and earnestly shows them the error of such thinking.
The second leader of men deserving the moniker of Dirty Dozen is Major Wright (first name unknown to me), another tough-as-nails man chosen for this impossible mission by the same man who green-lit Reisman's work, General Sam Worden. Wright's approach seems to be a tad more cerebral than rough-and-tough, at least until he is pushed and then he shows that brains and brawn are not mutually exclusive. He will be asked to lead men on two different missions.
The third man to lead a Dirty Dozen squad is Lieutenant Dan Danko. His missions, which number a dozen just like the members of his crew, supposedly take place a full year before the first Dirty Dozen took form - the reason for this discrepancy is not explained but his commanding officer was a different general so ...
In each case, the men who make up the convict contingent of each group will change because when the missions are said to be suicidal in nature, there was no hyperbole in play.
Note: in each incarnation of the Dirty Dozen, the members of the squad are military men, not spies, but since from the moment they shed their uniforms to go behind enemy lines to engage in the kind of missions they are assigned, the Geneva Convention clearly marks them as spies and saboteurs.