During the Cold War, we thought the next world war would have a nuclear exchange that had the potential to end all life on Earth. We now see that having nuclear weapons does prevent two nuclear powers from fighting a war directly with their nuclear weapons but it is fought through a 3rd country like Ukraine. We have seen this less clearly with Vietnam and Syria where the United States and Russia (or the old Soviet Union) weaponized each side. The situation is far different this time when it involves a European country. It is also different from the breakup of the Soviet Union. The United States and NATO didn't become engaged in this way when the Soviet Union invaded Hungry in 1956 or Czechoslovakia in 1968 because they were then fellow members of the old Warsaw Pact countries. We didn't oppose members of the Warsaw Pact invading each other. It is now different when many of those Pact members are now part of NATO. It is clear that Putin sees this process of those countries now aligning against Russian as a true "Domino Theory" similar to what we thought when we invaded Vietnam. The "Theory" is probably more real when the dominos are next door to your country than halfway around the world.
Even when you spend billions in aid to Ukraine is it a cheap way to wage a war with Russia and have the deaths only with Ukrainians and not American soldiers.
Can we imagine how different World War II would have been fought or turned out if Hitler had nuclear weapons?