Bucky Barnes was Captain America’s sidekick in World War II. Like Cap, he was preserved, only nobody knew about it because the Russians had him and used him as a brainwashed super-agent. Cap got Bucky unbrainwashed before he was killed. Now, Bucky is reluctantly Captain America.
Reading this as a fan of Bucky, it bothered me that yet another person was questioning him as to whether he was worthy. But now, after reading all these books featuring Clint, it makes sense. He has been really protective of Steve and his legacy. He just doesn’t know that he and Bucky have that in common.
Also, don’t pick fights with guys with cyborg arms. Bad idea.
Captain America Vol. 5 #37, pages 10-11 (details). June 2008. Collected in: The Death of Captain America, Vol. 3: The Man Who Bought America http://amzn.com/B005M48D0S