In 2010, the USA and Russia agreed to exchange prisoners. The political basis for such agreements is clear, but how does it work legally? Is there a formal agreement in international law between the US and Russia? Did the President have to commute or pardon the Russians in US prisons, or is there an existing statutory basis for releasing some prisoners?
Did the President have to commute or pardon the Russians in US prisons, or is there an existing statutory basis for releasing some prisoners?
As far as American law is concerned, in this particular case, it was a "simple" deportation.
The defendants pleaded guilty and were sentenced to time served, based on submissions from the prosecutors and the defendants, which are usually followed by the judge.
Alternatively, if the judge had sentenced someone to real prison terms, a commutation may be required. If they want, the prosecutors can also withdraw the charges before sentencing.
Then as "free" men and women who are foreign nationals, they can still be inadmissible to the United States and be deportable aliens under the Immigration and Nationality Act (8 U.S. Code § 1182 - Inadmissible aliens, § 1227 - Deportable aliens). In this case, there are so many potential grounds to choose from: conviction of a crime of moral turpitude, misrepresentation, national security, foreign policy, etc. The Secretary of State can also revoke their visas at the Secretary's discretion and make their presence unlawful.
They have the right to contest deportation if they are considered "free" as far as the criminal law is considered. But, they usually want to go home. The US may choose to prosecute them for other potential crimes if they contest.
Then if they are deportable and there is no stay on the proceedings due to judicial or administrative intervention, they can be removed from the United States.
Is there a formal agreement in international law between the US and UK?
I do not believe the United Kingdom was involved directly to the exchange itself. The prisoner swap was done in Vienna. Agreement needs to be obtained from the Austrian government regarding entry conditions etc.
UK revoked Anna Chapman's British citizenship. Igor Sutyagin and Sergei Skripal moved to UK. But those are not really international matters legally. The UK alone determines how its citizenship works and who can enter and stay in the UK.