The Budapest Memorandum (1994) gave assurances, that the U.S. would militarily intervene or defend Ukraine under attack like an alliance-treaty.

Ukraine surrendered the sharpest tool in its arsenal for those assurances, its inherited nuclear arsenal, the world’s third-largest at the time. But the loss was broader than warheads; it was the surrender of a strategic future.

America first means America first. All politicians will say one thing and do another, always check the incentives…

The Budapest Memorandum did no such thing. It is completely and totally incomparable to the US-Japan alliance. At most, it calls for a weaselly "security council action to provide assistance".

  >Seek immediate Security Council action to provide assistance to the signatory if they "should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used".