Tokyo Rose case - she, a US citizen, was arrested in Tokyo postwar and brought to the US for trial.
So there’s your precedent. If you are a US citizen, and the US is at war with Russia, and you persist in supporting Russia vs the US, the US can certainly try to arrest you while you are abroad. Your defense against arrest would be local laws and local politics.
John Kerry.
Anti war protests during the 60s.
Anti war protests during WW1.
Ah, more fascist globalist threats against American citizens to get in line.