The Budapest Memorandum on Security Assurances refers to three identical political agreements signed at the OSCE conference in Budapest, Hungary on 5 December 1994 to provide security assurances by its signatories relating to the accession of Belarus, Kazakhstan and Ukraine to the Treaty on the Non-Proliferation of Nuclear Weapons.
As a result, between 1994 and 1996, Belarus, Kazakhstan and Ukraine gave up their nuclear weapons. Until then, Ukraine had the world's third-largest nuclear weapons stockpile
If the US convinced Ukraine to give up their nuclear weapons with the promise they would help fend off an invasion, and then proceeds to not help fend off an invasion, what do you think happens the next time the US asks a country to give up their nukes?