Nuclear weapons have nothing to do with it. Putin, like Hitler, will not stop when he has got Ukraine/Czechoslovakia, he will see it as a sign of western weakness and press on to attacking and occupying other countries. Belorus is already a client state and will be taken over when its current dictator dies.
Transnistria, part of Moldova, Abkhazia and South Osettia, parts of Georgia , are partially recognised or unrecognised regions where break away groups, funded and recognised by Russia are similar to the Donetsk area of Ukraine and if Putin is successful in Ukraine, how long before he uses the same excuses to invade occupy and recolonise those countries as well. From there it is a step through Poland into the Baltic states, Latvia, Estonia and Lithuania. All four EU and NATO members.
Russian success in Ukraine, may well encourage China to invade Taiwan - and then we will be in World War 3, with wars in Asia, Central Asia and Europe.
Of course it is something to worry about, but when has anybody not be been born into frightening times? My grandfathers grew up into, and fought in World War 1. My parents were born during WW1. They grew up under the threat of Nazi Germany and fought in WW2. I grew up during the cold war. Do you remember the Bay of Pigs and other events when the whole world held its breath over imminent (within 24 hours) destruction in a nuclear war.
Since the Cold War we have seen the Baltic states riven and devastated by genocide, 2 wars in Iraq and the Syrian conflict. All of which could have brought the Western house of cards tumbling down.
When have ever lived without the fear of war. Surely we should instead by thankful that it is only man-made conflict that threatens us. Much of Turkey is in an earthquake zone and we have seen this week what damage a major earth quake can do. Most of California lives under the threat of a comparable event and many other places. Every year we see 10s of thousands die in natural disasters; immense floods, landslips and mudslides. We should be grateful that we live in a stable part of this earth where we do not fear geological disasters that can kill people in multiple millions.
We need to count our blessings.