The Avengers, Vol. 1 #117 (1973)

Avengers Defenders War - Holocaust
Dormammu is fuming at the involvement of the Avengers. Valkyrie easily bests the Swordsman. The first real breakthrough in the combat comes from Captain America and Sub-Mariner, who are able to communicate to a degree whilst in combat, and begin to realize that both sides may have been manipulated from the start.
Release Date: 1973