Is it legal for a U.S. citizen to fight for a foreign entity? Yes, under certain guidelines and restrictions. I will analyze the relevant statutes and attempt to answer the question through the use of a particularly interesting vignette featuring a U.S. citizen who has gone abroad to fight in a foreign conflict.
Mr. Dean Parker is a 49-year-old resident of Florida who has gone to fight against the Islamic State with the Kurdish People’s Protection Units (YPG) in Iraq and Syria. Parker left his job as a commercial painter and traveled to Syria with no prior military experience. He was motivated to help the Kurds in their fight against the Islamic State after watching a report about their struggle. There are a few federal statutes that apply to Mr. Parker’s situation: 8 U.S.C. § 1481, 18 U.S.C. § 959, and 18 U.S.C. § 960.
Together these statutes establish that a person within the United States cannot (A) enlist in a foreign service from the territory of the United States, unless they are a citizen of a foreign country that the U.S. is at peace with, (B) fight against any entity that the United States is at peace with, (C) fight with any entity that is engaged in hostilities against the U.S., or (D) serve as a commissioned or non-commissioned officer in a foreign service.
It is not illegal for a person to enlist in a foreign service once outside the territorial boundaries of the United States. At face value, Mr. Parker’s actions seem to fall within the statutes. He is not an officer in the YPG and is enlisted with a non-state actor that is largely perceived to be a U.S. ally fighting against a non-state actor who has directly threatened the U.S. Additionally, Jennifer Psaki, State Department spokesperson, commented on the few U.S. citizens that have gone to fight against the Islamic State in a State Department Daily Brief on October 2, 2014. She said that she was not aware of any law barring a U.S. citizen from joining a foreign military organization that is not designated a terrorist group.
There are a few complications. The YPG is not a designated terrorist organization but it is loosely affiliated with the Kurdish Worker’s Party (PKK), which the United States has designated a terrorist organization. The legality becomes murky here because 18 U.S.C. § 2339B clearly prohibits any U.S. citizen from being involved with any terrorist organization. Parker will no doubt be questioned extensively upon returning to the U.S., and could face the consequences if his actions are proven to aid the PKK in a way that falls under the statute.
Our current international system is defined by the interaction of sovereign states. Parker is fighting for a non-state actor that the U.S. can only be at peace with through legal ambiguities. The Kurdish people are spread over Turkey, Iraq, Iran, Syria, and small parts of Armenia. Though the combat occurs in Iraq and Syria, the fact that Parker is fighting for a people who span five countries could create complications for him in the context of our international legal system.
The ongoing civil war in Syria may also have an effect on the legal status of Parker because of the unstable environment and the constantly shifting alliances. Parker likely does not know of the entirety of the activity that the YPG is involved in and may find himself vicariously associated with other groups that the U.S. does not see as favorable as the YPG. If he finds himself involved in a battle where war crimes are committed, he could be prosecuted upon his return to the U.S. under the precedent of United States v. Belfast. This is a landmark case that found Chuckie Taylor guilty of human rights violations in Liberia and was the first time a person was prosecuted in the United States for human rights violations abroad. 
There are examples of U.S. citizens fighting in foreign militaries that are less legally uncertain. Thousands of Jewish-Americans have gone to Israel and joined the Israeli Defense Force, which is perfectly legal because of the U.S.’s allied relationship with Israel. Although it becomes more complicated when a U.S. citizen chooses to fight for a non-state actor against another non-state actor, it can still be lawful, as in Dean Parker’s case.
 Rebecca Collard, Fight Against ISIS, Time Magazine (Jan. 20, 2015), http://time.com/3674281/isis-us-kurds-iraq/.
 8 U.S.C. § 1481; 18 U.S.C. § 959; 18 U.S.C. § 960.
 Erin Creegan, Article, National Security Crime, 3 Harv. Nat’l Sec. J. 373, 394 (2012).
 Jen Psaki, State Department Daily Briefing (Oct. 2, 2014), http://www.c-span.org/video/?321857-1/state-department-briefing&start=NaN
 18 U.S.C. § 2339B.
 United States v. Belfast, 611 F.3d 783 (11th Cir. Fla. 2010).