Yes, but that power only extends to the government itself. The subservient American media will probably fall in line even though they aren't legally obliged to, and the rest of the world will mostly ignore him because this is obviously stupid.

I think the more important thing here is to understand that this achieves nothing. It's not useful as a threat - we literally do not care - and so only serves as deliberate antagonism. Given that America is still allegedly trying to get a trade deal with us, even if they're trying to force a more favorable deal with tariffs and other forms of pressure, this still doesn't help them reach that goal in any way. Like so much of what Trump does it's just childish and self-destructive.

Oh, I'm sure the president can decide what name America official calls something. That doesn't change the name of the actual thing. It will still be lake ontario

Never heard of it