Air Force Captain Nathaniel Adam was framed and convicted of treason. To avoid the death penalty, Adam took part in a military experiment where he was wrapped in an alien alloy and then exposed to an hydrogen bomb. The experiment blew Adam into the "Quantum Field" where he reappeared twenty years later, with tremendous super powers.
The government helped him adjust, making him into Captain Atom, a super-hero under their control. Captain Atom eventually rebelled against them, and joined the Justice League International and Justice League Europe.