Legitimate question, please, on the rules of war with regard to Trump's social media craziness [View all]

Okay, I don't know the answer to this, so I'm hoping someone else here does.

Trump indicates in his lunatic post that he's going after power plants and bridges on Tuesday. I thought that was "illegal" (knowing full well that legal means nothing to Trump the asshole).

But what makes going after infrastructure, etc. illegal? Is it part of the Geneva Convention? Rules of War? What?

And if it is indeed against some rules in place that supersede just the US Congressional area of control, then cannot another country step in and start proceedings to stop him?

I realize this is probably some obvious concept that I should already know, but I don't. I don't think I've ever had to consider the concept before (in my defense).

Thanks.