This is a fine answer. We, in the U.S., defend each other. Job #1 of a country is to defend its people.
As for Great Britain, same answer. We’re in a mutual defense agreement, reinforced by decades of us standing soldier to soldier. Plus, the U.K. meets its NATO obligation of spending of 2 percent on GDP.
As for Ukraine, I agree with you. It’s not the same thing. They’re not in NATO. Our support for them is based on pragmatic considerations. We might do for them what we did for Hungary in 1956, or Czechoslovakia in 1968. Russia’s invasions of those countries and of Ukraine were or are wrong, but we’re under no moral obligation to support them.
Yes, we made some assurances to Ukraine to induce that country to give up nuclear weapons, but Russia hasn’t used nuclear weapons on them.
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,