In 1994, the US played the major role in persuading Ukraine to give up its nukes; if Ukraine still had its nukes Russia would not dared to invade, so to my mind the US bares some moral obligation to help Ukraine resist the Russian invasion.
en.wikipedia.org
The real question is, will NATO have the will to fight if Russia attacks a NATO member state, or will Putin believe NATO will have the will to fight if he invades a NATO member state if the US and Europe show a lack of resolve in Ukraine? Would Putin have invaded Ukraine if he had known he would be facing this tough a war? Very doubtful. If the US abandons Ukraine, thus abandoning all the values and principles we were taught in school the US stands for, Putin will surely test NATO's resolve to go to war with Russia to protect Romania or Bulgaria, but will he actually invade them? No one can say for sure, but what we can say for sure is that as long as Russia is bogged down in Ukraine, or better if Russia is forced to retreat from Ukraine, Putin will not be in position to challenge NATO.