After his ship was sunk by a German U-Boat in the Arctic, Joe Colton spent several days adrift in the icy water. He eventually washed up on the shores of Iceland, where he stayed, frozen in the ice until he was found by Cobra in 2003. After learning the outcome of the war, he joined Cobra, taking the name G.I. Joe in honor of his fallen brethren and was quickly made field commander of the Cobra forces.
From the files of Cobra Commander: "G.I. Joe is a skilled soldier and a big boost for troop morale, as he is our only living link to what once was the United States."
I had another pair of pants that were more WWII style I was going to give him, but I decided that since he was with Cobra now, they probably would have given him some updated equipment. I decided to keep the basic shirt, though. I couldn't find a gun I found suitable for him.