Indy Is Back

So the new Indiana Jones and the Kingdom of the Crystal Skull teaser trailer is out and Mr. Ford looks like he's turned back the clock. I'm a huge fan of the series and I wish Hollywood would make more old fashioned movies like this. Remake some of the old serials and play them straight- I'd say they'd be looking at box office gold.

Anyway, can someone please explain to me why Indy is fighting American soldiers?