I had to defend some ground troops in this mission. I figured out that destroying the bridges appeared to help slow down enemy ground forces but I think I ignored too many of the enemy planes and choppers. So, I ended the mission with only 25% of the friendly troops remaining.
I think I could have done better, but I was frustrated with the mission and was happy to move on.
Incidentally, it's really quite neat to fly through some canyons, take out a bridge with a missile, have it fall out of your way as you zip by, and note the enemy tank that was on the bridge also falling to its doom. Not huge drama, but fun.