Placed on the roster by order of the U.S. government in West Coast Avengers (2nd series) #45. Fired in #69 after team reorganized under U.N. jurisdiction. Accepted onto the roster on his own merits as replacement for Hank Pym in #74.
John Walker was originally a freelance patriotic symbol, holding rallies to get the public to nominate him as the new costumed symbol of America over Captain America. He was chosen by the CSA to replace Rogers after the latter resigned in Captain America (1st series) #333. When Rogers returned, Walker was given a new identity as Jack Daniels, the USAgent in #354.
As the Super-Patriot, Walker used American throwing stars and a Liberty's Torch that produced a stream of flame. As Captain America he used Cap's traditional steel / vibranium alloy shield, before switching to a pure vibranium shield as USAgent. With Force Works, he adopted a photonic shield projector, producing an energy shield from his forearm that could discharge as an energy blast. With the Jury, he adopted their patented flight disks for aerial propulsion, and a eagle badge shield that could fly under its own power using control studs in his glove. With STARS, he began using an energy truncheon to stun opponents at varying levels and a photonically-reinforced eagle shield. With the Invaders, he wore a light-armored costume, using a machine gun, grenades, and a discus shield with retractable spikes on the circumference.
Recently, Walker was in a wheelchair following a battle with Scourge and the Spear of Odin in Thunderboltd (1st series) #142-143. He has lost a leg and hand, but had them reconstructed using artificially-manipulated symbiote bio-mass in #188. Whether this means his limbs will deteriorate when exposed to sonics or flame is unknown.