The lame way is to have Mr. Johnson send them to recover a protoype item from an aircraft that was brought down by a SAM over, say, Bogota. First priority is to find the crash site, and after finding it (and clearing it of hostiles), try to determine where the item was taken... and you're off to the races in a rough part of the world, with a reason to be here. Sort of a lame premise, but it gives you that excuse to be here, and can be modified a number of ways to make it more interesting.
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True. I was trying to come up with a decent reason, but screw it. I'll just use yours. My group usually doesn't complain when they spot a lame excuse, stolen idea, or contrived plot device (which is whenever I use one. I steal plots from games, books, and movies all the time.) They tend to be less concerned with the actual plot and more concerned with how many holes they can shoot or explode in things.