Walk through the evac site until you get to the exit. Follow Foley outside for one heck of a charge. You can also take a look at the wreckage of D.C. If you want. Follow Foley forward along the trenches. Once you get close enough, you'll be ready to engage the enemies in what is apparently the Department of Commerce. Wait for the signal and then break to the left. Follow Foley up through the door on the left. Rush forward and take cover behind the big desk or the ammo crates.
Use your M4A1 to clear out the balcony. Just keep up the fire. Foley and his men should be able to clear the floor. Advance a bit more and curve on over to the left. Advance into the courtyard and fire through the windows on the Russians inside. Take cover and then advance inside to the ammo crates. Don't rush in just yet. Move along the windows and fire through the windows at the Russians in the hallway. You might as well take care of them now. Also, you should probably trade out the pistol for a shotgun. It might come in handy for the close quarters up ahead.
Move down the hallway and take cover by the big desk on the left. There should be one more guy waiting in ambush down the hallway to the right. The door in front of you should also have one guy behind it doing blindfire. Send a few rounds through the door to take him out. Then quickly spin right and kill the Russian watching the stairs.

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Head on up another level. We're on the balcony to the entrance area, which should still be fairly clear. Watch the right side for any Russians you might have missed the first time. Then follow Foley down the balcony and up another level.
We have to do some more room clearing. Be very wary of Russians hiding in the corners as you clear the little offices. We need to cut out a little path to the enemy crow's nest. Just follow Foley or one of the rangers forward until you get to it. Get a full clip and look inside. Gun down the first 2 soldiers on the right, then curve to the left and see if you can hit a few more. Fall back to reload and then swing back in to take out any of the survivors.