Would it be legal for an American to go to Iraq and help fight against ISIS?

I think Okiknowit has the best answer but I'd like to add that during WWI American flyers traveled to Europe and fought the Central Powers with the French. They were known as the Lafayette Escadrille. In those days, I think it was much easier for the American government to turn a blind eye.

Perhaps it is time for something like that to happen again. It might impress on the public that the ISIS threat needs more attention than the government is giving it. Unfortunately, given the general idiocy of public opinion, they would simply be thought of as a bunch of nuts and they would be persecuted by the federal government if and when they attempted to return home. These days, the government insists on controlling everything and anything done by Americans overseas - and in the US too.

It would be legal but not a wise move

It may affect your Citizenship, and you wouldn't have the protection of the US Military, you would be a mercenary and the Kurd's may not be able to support you, and you will have to face their laws, and if you are in trouble you may not be able to get much aid from the State Department if things go wrong for you

Chetak

Dont worry we will be fighting ISIS within the next 3 years or so.

I am not a lawyer and cannot give you legal advice, but I once worked for a foreign government during a civil war in Africa, and I can tell you that the US State Dept will give you hell if you try. When I returned to the US for a visit and tried to renew my passport, DOS refused. But I cited the relevant law, called my Congressman and they renewed my passport the next day. If I were younger and did not have a family to care for, I'd join you in a heartbeat. I think, the Kurds are worth defending.

Even though 8 U.S.C. 1481 seems to prohibit service or taking an oath of allegiance to a foreign government or military service, the Supreme Court held in Afroyim v. Rusk, 387 U.S. 253 (1967) that you cannot unintentionally lose your citizenship by performing a potentially expatriating act. The Neutrality Act was also ineffective in preventing Americans from fighting in the Spanish Civil War.and Internal Security Act et al does not come into play unless you are actually fighting against uniformed American troops. The Agent of a Foreign Power Registration Act only affects you if you are performing services for a foreign government inside the USA. You might run afoul of the Patriot Act, since the PKK is technically still on the terrorist watchlist, but seeing as we are providing them with military assistance I doubt we are in a position to pursue it.

Further, you would be fighting under the flag of Kurdistan, which we do not recognize as a nation, and presumably would not have to take an oath that affects your allegiance to the USA. Check with a lawyer, but if you are sure to get a temporary export permit for any guns you are bringing with you, and get letters from the tax authorities and police in Iraq verifying you did not commit any crimes there, before you return to the USA. I think you will be OK.

The Kurds fighting against ISIS are actual military units generally formed along tribal lines. So I HIGHLY doubt they would allow an outsider in their ranks, and I know that Iraq would not (since technically the Kurdish military is part of the Iraqi military even though the Kurds beg to differ).

A US citizen a couple of years ago went to Syria to help the rebels that the US was helping. He had pictures and video of him firing RPG's at Syrian tanks. When he got back, he was tried and convicted byt the US DoJ for using a weapon of mass destruction.

So no, you can't do that and expect to either get in or come home when you are done.