The first part of your sentence is a valid statement: Not only is it in our own geopolitical interest to help Ukraine defend itself against the Russian invasion (Russia needs to be shown the folly of its ways and deterred from further aggression) - we are also under a moral imperative to make good on our assurances under the Budapest Memorandum.
The second part of your sentence is an unnecessary add-on which only clouds the issue / unnecessarily expands the topic at hand; Putin's use/non-use of nukes is not at all contingent upon the assurances we gave (or vice-versa).
Regards,
Were Russia to use nuclear weapons, it would soon become the largest ice skating rink ever. Drunks like Medvedev repeatedly threatens the west with the use of nuclear weapons. Putin, every now and then, hints at the use. But, the purpose of nuclear weapons is purely deterrent against being threatened or actually attacked with nuclear weapons. In a world in which other nations have nuclear weapons, nuclear weapons are kind of useless.
Ukraine lost this deterrent when it gave up its arsenal of nuclear weapons; and, the U.S. and U.K. gave assurances to Ukraine. Supposedly Russia did also, but that was always the fox giving assurance to the chickens. France and China, the other two legitimate nuclear powers under the Nuclear Non-Proliferation Treaty (NNPT), joined us in a watered-down assurance.
Now, as to what level of moral obligation has the U.S. to Ukraine. The Budapest Memorandum isn’t a Senate-approved treaty. It is a so-called executive agreement. I’m sorry, but Presidents don’t have any such power. In this country, treaties must be approved by two-thirds vote of the Senate.
A President, such as Biden, if he wants to act on the something like the Budapest Memorandum, would have to gain the acquiescence of Congress. It is not unusual for Presidents to cave into Congressional demands in return for such acquiescence. Biden and/or the Senate Democrats don’t want to do that. The now long delay in securing additional aid for Ukraine indicates that the Democrats miscalculated or else securing addition aid for Ukraine isn’t as high priority for them as continued illegal immigration.
The House of Representatives has approved aid to Israel along with cuts in IRS spending. That bill is dead in the Senate.
The House of Representatives has signaled a willingness to approve aid to Ukraine combined with Republican-style border security in the U.S. The Senate first tried to combine Democrat-style border security (more money for refugees) with aid to Ukraine, then passed a “clean” bill for aid to Ukraine. That bill is languishing in the House. It’s not dead. But, we’re not seeing a discharge petition.
Things would be different if Russia invaded the U.K., a NATO ally. Then under a Senate-approved treaty, a moral obligation would kick in. As FDR put it in asking for a declaration of war after Pearl Harbor, a state of war already existed, implying that regardless of whether the Congress declared war, he would act as Commander-in-Chief to defend this country.
As it is, no such moral obligation exists with regard to Ukraine. U.S. aid to Ukraine is a political matter.