To learn more I looked into a case I came across that had a fair amount of publicity and was resolved fairly recently. It concerned a crime committed in the United States, an alleged criminal with dual U.S./French citizenship who fled to French territory.
In 2007 Hans Peterson turned himself in to French authorities in St. Martin, Guadeloupe (a French territory in what is called DOM-TOM). A warrant for Peterson's arrest had been issued in the United States concerning the 2006 death of an American doctor, David Cornbleet, in Chicago. Peterson was alleged to have tortured and killed him - a very violent crime indeed.
The U.S. authorities requested his extradition and they were refused. Why? Because the French determined that Peterson was a French citizen and, in the words of a spokesman from the French embassy, France does not extradite its own citizens to face trial in other countries no matter what the penalty and that this is a matter of French law:
“It’s not a decision by our government, it’s just a legal provision in our law,” he said. “We can’t do it. We’ve never done it.”
So what about that extradition treaty that exists between the U.S. and France? Read the fine print. It says in Article 3:
"1. There is no obligation upon the Requested State to grant the extradition of a person
who is a national of the Requested State, but the executive authority of the United States
shall have the power to surrender a national of the United States if, in its discretion, it deems
it proper to do so. The nationality of the person sought shall be the nationality of that person
at the time the offense was committed."
Or as IntLawGrrls puts it, this treaty is based on a template that includes certain standard stuff:
"...a pledge of mutual obligation to extradite fugitives in certain serious criminal matters; detailed procedures for such transfers; and exceptions from extradition, for certain offenses of a political nature, and for capital offenses absent assurances that the fugitive will not be subjected to the death penalty if convicted.
But it also says that neither country is under any obligation to surrender its own nationals. The U.S. could do it if they wished. France says they couldn't do it even if they wanted to. Interesting.
Does that mean that French citizens (or dual U.S./French citizens) can escape justice if they flee to French territory?
No, it just means that France will try that citizen in her own courts on her own territory. That is exactly happened to Peterson. He was tried in France, convicted and originally given a sentence of life in prison. On appeal in 2012 the Cour d'assises de Paris reduced this to 30 years: 20 years in prison and 10 on parole.
These are the kinds of situations that cause tensions between countries. For some Americans, it may seem that Peterson got away with murder which is not true. He was simply judged under his other country of citizenship that convicted him and applied penalties under her own law. Had he been tried in the U.S. would it have gone differently? Hard to know what a U.S. court would have said. Peterson is clearly mentally unstable and who knows what defense his lawyers would have used. What he avoided completely was any risk that he would be tried and sentenced to execution for his crime since France abolished the death penalty in 1981. Interesting enough so did the state of Illinois in 2011. So he would not have faced that penalty in any case.
As for the U.S. government's reaction, when France informed the U.S. that they would not turn him over, the U.S. senators from the state of Illinois got involved and wrote to the French asking that they reconsider. A delicate diplomatic situation. In the end, of course, it didn't happen and hard to see what the U.S. could do about it under the circumstances.
There you have it - an interesting example of how treaties, dueling sovereignties and conflicts of domestic laws lead to a particular outcome when it comes to dual nationals and extradition.