It's no problem.  It doesn't really have to be anything specific, I don't think.  Imagine a bunch of pickup trucks with recoilless rifles and heavy machine guns, and bad guys with RPGs and AKs, just seeing a group of 3 or 4 humvees rolling across the desert, or a small outpost, or a flight of helicopters, and deciding to make an aggressive push to take them out.  And then our characters pop out and are like "Welp, we're boned.  Time to leave."

Sorry if that sounds a bit dark.  I do like your idea as well, but I'm trying to think of how to work something like that out.