Avengers Assemble Vol. 2 (Avengers (1998-2004))
Collects Avengers #12-23, Avengers Annual '99, & Avengers #0:

Threats including the Thunderbolts, Dominex, Lord Templar, Pagan, and the Wreckin Crew! Then: every man, woman and child in Slorenia is dead with a message from its murderer: Ultron has returned! Our only hope is a desperate U.N. strike on Slorenia, led by the Avengers themselves.