General Discussion

But (first) he wasn't claiming it for himself. He was asking for it to give as a gift from the US to the British monarchy. The property of the US, including historical artifacts can be given as diplomatic gifts to leaders of foreign countries. This is not unusual. Follow the links I gave you, or do some research on your own.

(second) the head of the library can't be an accomplice if the sword was being given as a diplomatic gift - any more than when a decision is made to give any American artifact to a foreign dignitary as a diplomatic gift

(third) was it stealing when Biden took a leather folder of letters between Queen Elizabeth and Eisenhower to give King Charles as a gift in 2023?

The giving of US property to foreign dignitaries as diplomatic gifts is absolutely ordinary.

Trump's threats are trump's threats, Trying to coerce officials by the use of threats is just Trump being Trump. Reprehensible - but it doesn't turn an ordinary diplomatic act into attempted theft.