all land units retreat to a single adjacent territory, while the air units remains in the territory.
The only way you did something illegal is if you retreated your air units somewhere, and then moved them to S. Italy.
Can a plane whose aircraft carrier has been damaged or destroyed land in a friendly neutral country. e.g. GB1) british fighter participating in a battle in SZ 97 was planning to land in a carrier within the same SZ. The carrier is damaged. Can this fighter land in Greece to complete its move. Does it matter whether Britain activates it with a land unit in its non-combat move phase of turn 1.
To be able to land a plane in a territory, the territory has to be under controll by you or your allies at the begining of your turn.