Ukraine would not need help to defend itself if it had not trusted the US and gotten rid of its nukes, and the US is not at war with Russia any more than it was when Reagan gave Stingers to the Afghanis to fight the Russians. There is no rational basis in fact or logic for claiming the US caused this war; clearly Putin's imperialist ambitions are the cause of the war.
In fact, Russia only constituted about half of the USSR in population and economic power so it would have to conquer a lot of countries to reconstitute the USSR empire, and several of those countries are NATO members, so do you think the US shouldn't be trusted to live up to its obligations under NATO just as you think the US shouldn't be trusted to live up to its obligations under the Budapest Memorandum?