>>717692243
There's a scene in that same cutscene showing the UNSC is fighting similar battles across the entire continent, it's not that surprising they could be stretched a little thin.
>>717713389
How else would you surprise assault a large shielded force with AA stopping your frigates and bombers from getting close? It's not like there weren't Scorpions right behind. Small strike teams (your job in the mission) would just get swarmed by the covenant if they weren't distracted by the greater assault.