My friend and I recently started playing a&a 1941. I played the germans which attacked Caucasus then my friend started moving in units from other territories into Caucasus to defend. Can you do that?
First of all, a defender can't move sea units into the Caucasus. (An attacker can fire a one-shot bombardment from a battleship in conjunction with a land attack.)
The second thing is that a defender can't move units during another player's attack portion of the turn. A defender may move defenders into a territory during the portion of the turn when s/he is entitled to attack.