Telling a sovereign nation, and the US’s closest ally, it should become the 51st US state isn’t making war, but it’s way more than imposing tariffs in contravention of treaties this president signed in his last term.
It's just a joke. A bad joke, but just a joke. Republicans are afraid to add Puerto Rico as a state for fear of it voting Democrat, they would never allow Canada to become a state even if you wanted to (or rather, 10 states), because they would never win an election again.