An attack between units present in the same area can be resolved by just dragging and dropping the icon of a unit over another. See [URL=/help/unit/combat]Combat[/URL] for more information. Using the action is necessary for ranged attacks, between two distinct areas. This includes bow and crossbow attacks, as well as destruction of buildings with catapults or other siege engines. In that case, you must select the attackers and attack the area; you can't target a specific unit.