True life I want to fight isis: Isn't that illegal?

While a 2 century old law states that it is not illegal to fight for another country, it does state that US citizen has to be recruited in that country. So wouldn't going there be illegal, and if not, I'm pretty sure trying to transport a barrett definitely is. (Robert's probably not going to take it, but why even talk about it?) Even to do anything remotely to this, they would have to fly to Miami or Washington, then to somewhere like Norway to even get close, which would be kind of hard, airlines do double takes over nail files, so how could any of this go down without someone being arrested going there or coming back, especially when it's now on tv.