Ukraine had nuclear weapons, but gave them up. If they manage to reconstitute one, would it count?

Yes, that would count.

How do you treat NATO stationing nuclear weapons in a country (like Poland is requesting)

@DavidFWatson An extremely good point raised here.

Any new nation would need to have sovereign control of nuclear weapons to count as "possession" under this question. Under current NATO agreements, the US "maintains absolute control and custody of their nuclear weapons forward-deployed in Europe." For the purposes of this question, NATO stationing of weapons in a new country under current agreements does not equate to possession. https://www.nato.int/cps/en/natohq/topics_50068.htm

Similarly, stationing of Russian nuclear weapons in Belarus doesn't count as possession given those weapons remain under Russian control.

Does there need to be any particular degree of credibility or evidence to go with the government statement?

@EvanDaniel No, but it does need to be an official statement from the government stating unequivocally that the nation possesses nuclear weapons. This covers the case of, for example, the US providing South Korea weapons where they would not need to perform a test for credibility.

Confirming this by Jan 1st, 2025 - before the US may change presidents?

@mattyb Good call, I have modified closed date to 1/1/25. Mistakenly had 12/31/25 here