Title: CAPTAIN AMERICA#354 by Mark Gruenwald (writer) and Kieron Dwyer (artist)
Synopsis: A government agent dressed as a Watchdog pretends to kill Captain America VI so that John Walker can take on the identity of US Agent. Unfortunately the agent also manages to fool Scourge II who now returns to action. Dressed in standard battle garb, Scourge guns down the agent before escaping
Is it key to the overall Scourge storyline? Well, it's the return of Scourge II after a lengthy absense. At the time the return didn't seem so dramatic since it was then possible that he was the Scourge the Red Skull was using
Is is part of the main story in this issue? Only in the sense inadventently helping the government's deception, which is followed up more in later issues of this title and West Coast Avengers.
Comments: Scourge II was probably lying low to take the heat of Captain America's search. It's possible that Scourge III's actions resulted somehow in Scourge II returning to action.