Thirty-year-old U.S. Army veteran Eric Harroun was charged Thursday with violating U.S. law by fighting with Syrian rebels to depose Bashar al-Assad.

Harroun was charged with "conspiring to use a destructive device outside the United States" after telling FBI agents that he used rocket-propelled grenades against Syria's military. Harroun entered Syria in January and ultimately joined up with Al Nusrah fighters, which the U.S. says are aligned with al-Qaida. He boasted about his war exploits on Facebook, including the downing of a Syrian government helicopter.

An affidavit drafted by FBI agent Paul Higginbotham reads: "There is probable cause to believe that, in or about January 2013 to March 2013, Eric Harroun conspired to use a weapon of mass destruction, i.e. a Rocket Propelled Grenade, outside of the United States, in violation of 18 U.S.C. 2332a(b)."

Harroun is not currently charged with using a weapon of mass destruction, but the charge could stick, says Ohio State foreign and defense policy expert John Mueller. "I think they've got him," Mueller told U.S. News. "The [law] puts RPGs right up there with revolutionary war muskets and bombs bursting in air."

Mueller noted in a 2009 Foreign Policy article that the definition for "weapon of mass destruction" in 18 U.S.C. 2332a includes "any destructive device as defined in section 921 of this title." The definition for "destructive device" reads:

(A) any explosive, incendiary, or poison gas --

(i) bomb,

(ii) grenade,

(iii) rocket having a propellant charge of more than four ounces,

(iv) missile having an explosive or incendiary charge of more than one-quarter ounce,

(v) mine, or

(vi) device similar to any of the devices described in the preceding clauses;

(B) any type of weapon (other than a shotgun or a shotgun shell which the Attorney General finds is generally recognized as particularly suitable for sporting purposes) by whatever name known which will, or which may be readily converted to, expel a projectile by the action of an explosive or other propellant, and which has any barrel with a bore of more than one-half inch in diameter; and

(C) any combination of parts either designed or intended for use in converting any device into any destructive device described in subparagraph (A) or (B) and from which a destructive device may be readily assembled.

Under the WMD definition, "large-bore muskets fit, and it looks like a potato gun would as well," Mueller told U.S. News. "The potato would be a projectile expelled by a propellant, and most potatoes I have come to know are considerably more than a half-inch in diameter. The 'projectile' does not seem to need to be explosive (or even hard), and the propellant does not seem to have to be explosive, either."

University of California, Berkeley law professor John Yoo, who worked in the Justice Department's Office of Legal Counsel during the George W. Bush administration, doesn't share Mueller's cynicism about the law.

Yoo said he doubts that a rocket-propelled grenade would actually be considered a weapon of mass destruction under U.S. law, pointing to definitions in 50 U.S.C. 2902 and 50 U.S.C. 2302 that exclusively identify chemical, biological or radioactive weaponry.

"DOJ is using an extraterritorial criminal ban on using weapons larger than standard handguns and rifles, but I would not call it a weapon of mass destruction," Yoo said.

Al Nusrah, the group with which Harroun was associated, is classified by the State Department as an "alias" for al-Qaida, making it a crime for Americans to provide assistance to the group. The New York Times reports that Harroun told the FBI, however, that "he hated al-Qaida, that he did not know any al-Qaida members, and that he would fight against any regime if it imposed Sharia law in Syria because he was opposed to all forms of oppression."

Fox News reports that Harroun—now facing a life sentence—was arrested Wednesday near Dulles International Airport in northern Virginia. He had voluntarily returned to the U.S. via Turkey for FBI questioning.