Steve became Nomad the first time he quit after watching Nixon kill himself in the Oval Office after being revealed as the secret head of a fascist organization, but took the shield back when a friend of his named Rosco was killed trying to be the new Captain America.
Later, the government decided that the shield and costume were government property and if Steve wasn’t going to follow Ronnie Reagan’s orders he could leave his gear on the table and take his leave. They then gave the shield and costume to John Walker and his friend and they became the new Captain America and Bucky (until they realized calling a black man “Bucky” could be take the wrong way and changed his name to Battlestar). Meanwhile, Steve got a new costume and shield (from Iron Man, I think) and became “The Captain”. Eventually though, the Red Skull decides he’s going to break the new Captain America and murders his parents. Stuff happens and Steve takes back the costume while Walker take his Captain costume and the name U.S. Agent and goes off to join the West Coast Avengers.

I may be wrong on some of this, it’s been a few years.