What are you talking about? Do you even know how wars are fought? Chucking it to stupid plot development is stupid in itself.
How many troops you have directly influences the outcome of a battle. You can win battles with less troops than your enemy but even a small enemy reinforcement can change the outcome.
Also, commanding your troops is paramount, leaving the battlefield even for 5 minutes can be catastrophic.
Also, yes, you can fly troops in when you control only a few city blocks.