There are points specified on the map where you should build flaks to increase the chance of stopping the "invasion". However you won't be able to build them all. If you don't recruit soldiers, you will be able to do it, but rangers will eat you alive. The only thing you can do is create a half-assed defence xD.
Even if you don't recruit soldiers, after you "wake up" all the crew you will have even less population available. If you delay to "wake up" those soldiers or ignore the "flies", airborne guys will seriously mess the place, spamming mines, attacking neutral soldiers and grabbing weapons.
Play it for fun. Some users might say that Tiger Ace is the best campaign of ToV, but for me is Trun.
If you are playing it for the medal and don't want to cheat, i can give you an advice... what i did to win the medal without "cheating" (perhaps wasn't the best method but worked):
Save every three minutes XD... if something goes wrong, reload the saved game and you will know what is going to happen.
It's difficult because allies use arty, strafe and bombs runs as if were free but your tanks will survive... or not? xD.
Mors Indecepta
Might controls everything, and without strength you cannot protect anything. Let alone yourself...