Now this is a strange and interesting case. You may have seen an interesting post recently at Foreign Policy describing a US citizen (and former soldier in the US Army) named Eric Harroun, who appeared to have gone off to fight against the Assad regime in Syria, possibly falling in with al Nusrah (an outgrowth of AQI that has been prominent in the fighting). Well, Eric Harroun is back in America now, and he is being prosecuted.
The criminal complaint and accompanying FBI affidavit are here. For now, the charge at issue is 18 USC 2332a(b), which in relevant part makes it a crime for a US national to conspire, while abroad, to use (among other things) a rocket or a weapon with a bore of greater than a half-inch diameter (don't be thrown off by the fact that 2332a refers to "weapons of mass destruction"--the statutory definition for this purpose reaches far beyond CBRN weapons, and indeed far beyond the lay understanding of "mass destruction"). As far as can be gleaned from the affidavit, it appears that the hook for using this charge is that Harroun trained on (though did not apparently use) an RPG while in al Nusrah's camp.
Lots of interesting issues here. Of course it highlights the delicacy of the United States or other governments finding the right partner when we seek to provide aid to anti-Assad forces. As for the charges available in this particular scenario: It's not immediately obvious that this is a great fit for a 2332a(b) charge, unless there's more to the RPG connection or more involving some other qualifying weapon, given that we are talking about training but not using an RPG. I bet there have been some fascinating debates about whether to rely as well on a Neutrality Act charge, such as 18 USC 960 (query whether the US is properly described as "at peace" with the Assad regime!). I would think 18 USC 2339D (receiving military-type training from a designted foreign terrorist organization) would be a perfect fit here, at any rate, and am a bit surprised not to see it.