It's hard to strip a natural-born American citizen of their citizenship.

In fact, unless they renounce their citizenship, it doesn't look a lot like it can be done. There is a law that says if you serve in a foreign army, you lose it, but that's been ignored mostly (eg, those who served with the RAF before the US was at war with Germany, and those who served in China against Japan before the US was at war with Japan.)

It'll be damn hard to convict Walker as a traitor. (Two witnesses to the traitorous acts are required by our constitution.)