According to Marvel, Captain America, a soldier, opposed the use of atomic weapons against the Japanese so much, that the eeeeeevil US Government killed Bucky, froze Cap, and cloned him, since he would have opposed this action that did save countless American lives.
But of course we don't know if this really happened, since Marvel had the story written in such a way that it may have been a series of images created by a villain, who hoped to manipulate Cap's mind.
