Trump has betrayed allies, broken treaties, and damaged our standing with the world. He's also committed treason. He displays no diplomatic skills other than being a bully.
I realize that a lot of Trumpers admire him for being a "tough guy" (or, at least, pretend to). But his bullying behavior is actually a sign of weakness, and every other country knows it. You in fact do NOT get a good deal--in business or diplomacy--by being aggressive and refusing to compromise. But Trump only knows that one strategy.
I am HIGHLY amused by Trump crowing about his success in creating new treaties and trade deals that were only necessary because he destroyed the old ones (as Trumper cheer). He doesn't have the slightest goaddamn idea as to whether the agreements he destroyed were better or worse than the new ones. He just wanted to kill them off because they were enacted by Obama.
The long-term damage is that both our allies and our adversaries will regard us as a nation that cannot be trusted. If we sign an agreement and then pull out of it as soon as Trump gets a bug up his ass, what good is our word? AND...why would any signatory to an agreement with Trump trust him to not renege on it?
And in the Middle East--hoo boy, you betray an agreement, you won't get a chance for a second agreement. They've got grudges there that have lasted for over a thousand years. Trump's damage to American relations with the Middle East may be permanent and irreversible.