1a) Protect the U.S. border station (You will have to protect the border station against a counter-attack. It shouldn't be too difficult, especially since you'll have plenty of cover points to hide your men. Focus most of your attention on the enemy humvee.)
Obviously rebel units will arrive from the south (#1), so that's the same area you've already explored during the first part of this mission. You should also know that there's going to be a mortar strike (#2), so you will have to hide your men near the checkpoint. Make sure that they're not standing too far from the main building. You will also have to act quickly, because you will only be given a few seconds before the attack will take place.
Like I've already said, you will have to place your team members in correct spots in order to increase your chances for a successful win. Order your Rifleman units to use two heavy machine gun posts (#1). As for the sniper, he should be hiding behind one of the sandbags. You, on the other hand, should enter the border station building. Wait for the attack to take place (#2).
You shouldn't have any problems with the first wave of enemy soldiers and your men will probably assist you during these fights (especially if you have a sniper with you). Sadly, it's going to get more difficult very soon. You'll probably notice a humvee heading your way. Take out its gunner AS SOON AS POSSIBLE (#1). You could also use a grenade launcher to destroy this vehicle. You will have to maintain this position for a few more seconds (#2). You should receive a proper confirmation that the border station has been saved.
1b) Collect reinforcements (This is strictly a formality. You will have to wait for one of the allied gunships to get here. You will be given a new team, as well as other toys. ;-))
Ignore the mortar assault, because most of the explosions will occur to the north and you won't have to go there. Instead, head south. Obviously the rest of the team should be following your lead. You will have to get to the reinforcements area (#1). Now you must wait for an allied gunship to get here (#2). Just as before, you will command two new soldiers like they would be only one person (member of the Ghost squad).
2) Secure Mexican border station
This is going to be a very long and quite challenging run. Your objective will be to reach and secure the southern border station. You will have to eliminate everything that stands in your way. Gather your men around your current position and start moving south (#1). Make sure that your soldiers are using various objects for cover. This is going to be quite important, because you'll come across new enemy units very soon (#2). Try taking them out from larger distances.
You will also have to guard the left side of the bridge (#1), because some of the rebel units may try surprising you from the left flank. You would have to be ready for that. Make sure that you don't move too quickly and that your men are hiding behind cover. You should be using cars for cover, because smaller brick walls won't do the trick here. The second group of enemy soldiers should appear once you're close enough to the truck (#2).
Check the first screen (#1). Once you've reached that area, you MUST stop. This is going to be very important. Open your inventory and choose a ZEUS rocket launcher. Start moving south (slowly). Your team is going to be attacked by an enemy gunship (#2).