God damn. So, the Budapest Memorandum is short enough to read in a couple of minutes: https://treaties.un.org/doc/Publication/UNTS/Volume 3007/Part/volume-3007-I-52241.pdf

It’s incredibly badly written. The most relevant part:

4. The Russian Federation, the United Kingdom of Great Britain and Northern Ireland, and the United States of America reaffirm their commitment to seek immediate United Nations Security Council action to provide assistance to Ukraine, as a non-nuclear-weapon state party to the Treaty on the Non-Proliferation of Nuclear Weapons, if Ukraine should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used.

What is “assistance”? Selling weapons? Giving weapons? Sending troops? Using nuclear weapons in retaliation? I definitely don’t read that as a security guarantee. But, it seems cunningly vaguely crafted in a way that could make it sound like one.

Also, do they even have to actually give “assistance”? Or just seek assistance from the Security Council and then they’re allowed to give up if they don’t find it?

Does it even get triggered by “victim of any act of aggression,” or does that clause about “in which nuclear weapons are used” also apply to that first contingency?

It’s basically so poorly worded that it could mean anything. Presumably, this was either a deliberate goal, or else the result of the various parties being so unwilling to come to an agreement that they just wanted to get something signed and all move on regardless of whether it meant anything, or both.

Of course, if we’re assigning any blame to Western powers for not stopping the Russian Federation, we should also be noting that the Russian Federation agreed to respect the territorial integrity of Ukraine, including the existing borders, and not to attack Ukraine under any circumstances. But we always knew modern Russia is so full of shit their eyes are brown, so no surprise there.