The faction that moves units into another region is always the attacking faction regardless of tokens that were already in that region.
1. Humans move into a region with an Air Field token and Trog War Party token and no other units. Battle is triggered because the war party token brings Trogs into the region. Humans are the attacker, Trogs are the defender.
2. Same as above, except no Air Field token. Humans are still the attacker.
To keep it simple just remember, if a battle is triggered the faction that is moving plastic pieces into another region is always the attacker. Tokens are irrelevant.
Remus is correct. Thanks!