What are the rules concerning mercenaries if captured?
[ihl-databases.icrc.org]
According to my own understanding of international law, mercenaries are the same as non-military persons from the belligerent nation who take up arms. Because they are not citizens of the belligerent, they do not have the protections that may entail. And according to US law, if you take up arms against a nation not at war with the United States your citizenship is at risk.