If one leaves the US to commit an act illegal in the US but legal in the country they travel to are they guilty of a crime?

Question

Before anyone asks this is part legal curiosity, part proving for 'loopholes' that I could have villains in a story exploit, I don't need 100% accuracy so much as general guidelines, and any unique situation or known 'loophole' is also interesting.

I'm curious about how the US handles travel to other countries to activities crimes that are illegal within the US but legal in the country they travel to. At first I thought that this would be legal, but I know sex tourism at the least is restricted, They actually make us take a training at work about how sex trafficking is bad in case we couldn't figure that one out on our own.

A quick scan online tells me that the sex tourism thing is specific federal law. Outside of sex tourism of individuals under the age of 18 are there other situations where one can be punished for traveling to another country to commit a crime? If so are these specific laws that address only a few unique situations, or is there some general law that addresses many crimes committed by traveling to another country?

I can think of a number of theoretical odd situations as fodder for story telling, I'll likely ask a few of them in more detail in later questions depending on the answer in this one. If someone wants to touch briefly on a few odd situations below in their answer I would be interested, but if any of these situations is too complex I'll ask them as a later follow up question.

1. how does dual citizenship work in situations where it would be illegal for a US citizen to travel to country X to do activity Y, but the individual in question is not only a US citizen but also a citizen of X?
2. How much does 'premeditation' apply to these rules? For instance my quick scanning of sex tourism law refers to an individual traveling to a country "with the intent to ...", does that mean if someone travels to a country and only then decides to do an activity it would be legal (ignoring the difficulty of proving it wasn't premeditated)?
3. Is a non-US citizen within the US (for instance with a visa, or even an illegal alien who lives in the US) still subject to these sort of laws if they leave the US with the intent of doing something illegal within the US and then return?

Edit: ...also Id love at least one other instance of an activity that would be a crime if someone travels elsewhere to do it. I'm thinking of potential loophole abuse questions I could ask, but I really don't want to ask them in context of the sex tourism law. Rather or not it's just a theoretical question to explore legal oddities I really don't like even hypothesizing the sort of situation that leads to someone trying to violate that particular law.

Answer 1

Like many US legal questions, there is a Congressional Research Service report about this.

It is not generally a violation of US law to do things in another country where the only connection with the US is that the offender is a US citizen. However, there are a number of general situations where the US has jurisdiction over federal crimes if either the victim or offender is a US citizen: if a place isn't within the jurisdiction of any country (e.g. Antarctica); a place used by a US government entity (like an embassy or airbase); crimes by American soldiers and those employed by or accompanying the military; etc. These are considered to be within the special maritime and territorial jurisdiction of the US.

Other laws apply if they say so. For instance, any US national committing war crimes inside or outside the US can be punished under US law; ditto for treason. The Foreign Corrupt Practices Act makes it illegal for a US national to bribe a foreign official anywhere outside the US for business reasons (if it's inside the US, there are more requirements). For instance, you aren't allowed to pay kickbacks to a foreign government's acquisition officer to buy your product. The CRS report has more (it doesn't include the FCPA, but that appears to be an oversight).

Note that extraterritorial jurisdiction doesn't just apply if the person is a US national. US laws can also confer it if the victim is a US national, if the offense has a significant US component, if it's directed towards the US, if it's in violation of international law and the offender later turns up in the US, etc.

For your scenarios:

1. Dual citizenship doesn't matter. A US citizen is a US citizen, and is required to obey all laws that apply to US citizens, unless those laws explicitly exempt dual citizens. A dual citizen isn't treated differently by the government; as far as the US government is concerned, their US citizenship is all that matters (except for certain specific purposes like security clearances). In Kawakita v. United States, a US-Japanese dual citizen was convicted of treason against the US for aiding Japan in WWII.

2. Depends. Plenty of these laws have no requirement that anything related to the crime actually happen in the US; for sex tourism, the subsection about traveling in foreign commerce for the purposes of engaging in illicit sexual conduct is followed by a subsection about engaging in illicit sexual conduct in foreign places. "Travel with the purposes of X" or "with intent to X" means you must have intended to do X at the time you traveled, but most extraterritorial laws don't control travel with intent to X (they cover X directly).

3. Depends on the law. Some laws (like child sex tourism) apply to any permanent resident of the US as well as any citizen. Some apply to anyone, because they're based on a conspiracy started in the US. Others apply just to US nationals; a noncitizen isn't bound by them (for instance, no one but a US national can be charged with treason against the US, for obvious reasons). Still others apply to anyone who later turns up in the US, even if that is literally the only connection between the US and the offense (this is basically reserved for crimes against international law, like genocide).