Trump is vindictive and petty. He would take revenge somehow. Whether it is just a very STABLE GENIUS tirade on lies.social or trade sanctions or a threat to invade the country is anybody's guess. But they're all on the table and he's got the eternal memory of an elephant. That's why every leader knows they have to go in with some honey first. They pay him a compliment or bring something he wants or likes. Then they can play hardball if they have to but you gotta appeal to the frail ego first. And that's why few leadership people will call him stupid in public or even to his face. They will hide the stupid medicine in some rhetorical sugar until their country is economically and militarily independent from a Trump government.
I am no leader, praise the heavens, and I wouldn't call him stupid. He's uneducated in many areas outside real estate development. He's got the interpersonal maturity of a 4yo. But he is street savvy and media savvy. There's a reason why he is this popular with his followers.
Constitutionally, it is spelled out in Article I, Section 10, Clause 1:
So foreign policy is principally a federal government domain, established by cases heard by the Supreme Court. https://constitution.congress.gov/browse/essay/artI-S10-C1-1/ALDE_00001097/
There is a somewhat gray zone where neighbors can talk to each other on the state level, e.g. Maine and Quebec. But they will find themselves restricted by what the governments one step up have decided. I think certain states also are on friendly terms with other nations, probably to deepen economic ties. But that's more on the level of a city friendship than actual foreign policy.