Adding to Trihero’s reply…
Aircraft that retreat also have to go to the designated territory. HOWEVER, if they still have flight range left, they can fly to someplace else to land in the non-combat phase, to the limit of their range for the turn.
Ex:
Germany attacks Caucuses and brings a FIG from Balkans along. After 1 round of battle, Germany knows they are going to lose and retreats to Ukraine. The fig has now flown 3 spaces 1 from Balkans to Ukraine, 2 from Ukraine to Caucuses, 3 from Caucuses to Ukraine in the retreat). In Germany’s non-combat, the fig can stay in Ukraine, of fly to Balkans, Eastern, Bel, or West Russia to land (assuming the designated territory was under German control at the start of the round of course).