They have already said they want to send US citizens there too.

https://www.huffpost.com/entry/white-house-confirms-trump-is...

They simply have to not validate you are a US citizen, no rights for anyone means no rights for everyone.

Habeus corpus is only suspended in war. Not a citizen doesn’t mean Russia can jail any American connecting in Moscow because they had a bad haircut.

It's not a slippery slope, it's blatantly unconstitutional already.