All allied units defend together - in the air, on land or at sea. An American carrier carrying a Soviet fighter and a British tactical bomber, being attacked by a German destroyer would have all three nations defending.
I know the context of the original question was w/all parties are at war, but this was a pretty broad statement so I felt the need to clarify (not trying to nitpick).
"All allied units defend together - in the air, on land or at sea" is true only if all parties are at war. Powers not at war can't be on their future ally's territories, carriers or transports (so this part holds up because it can't happen). At sea however, a neutral power could share a sea zone with its future ally, and the enemy could attack and ignore the neutral power (so they won't defend together).
I know the context of the original question was w/all parties are at war, but this was a pretty broad statement so I felt the need to clarify (not trying to nitpick).
"All allied units defend together - in the air, on land or at sea" is true only if all parties are at war. Powers not at war can't be on their future ally's territories, carriers or transports (so this part holds up because it can't happen). At sea however, a neutral power could share a sea zone with its future ally, and the enemy could attack and ignore the neutral power (so they won't defend together).
from http://www.axisandallies.org/forums/index.php?topic=28562.msg1028985#msg1028985