Something I got from quora on how this started.
So check, Ukraine was apart of the Soviet Union. Soviet Union fell and because of where Ukraine is, they had hella Soviet nukes. A treaty in Budapest was signed in I think 94 or 95 where they gave up their nukes (third largest stockpile in the world at the time) on the strength that the UK, US, and Russia would not threaten their territorial sovereignty (also applied to Belarus and Kazahkstan).
Since then Ukraine has been split between moving closer, economically for the most part, to the EU or to Russia.
Until 2014 nothing really happened. Some slick talk but nothing serious. Then the Maidan revolution took place in Ukraine in early 2014. This resulted in the overthrow President Yanukovych, who wished to push Ukraine closer to Russia and refused to sign an EU agreement on trade. Poroshenko was then elected overwhelmingly, on the platform of bringing Ukraine into more agreements with the EU. While this is happening, in the East of Ukraine, two regions, Donbas and Luhansk, moreorless broke away from Ukraine and were running shyt as independent states. The east wanted to go with Russia, the west wanted to go with the EU, basically.
Ukraine was in a civil war of sorts, with a ceasefire for a couple years now. Russia has admitted to providing some aid to the eastern Ukraine regions that broke off, but thats all they copped to. In reality, Russia was on the ground handling shyt in unmarked vehicles and unmarked troops (call the little green men). So the whole east side as been dug in for a minute. Brings us to today, with Russia popping real shyt. Oh and Crimea in the south of Ukraine became apart of Russia during the "civil war". They voted to join Russia via referendum that was like 90% to join Russia. Straight bullshyt but the UN didn't do shyt but condemn Russia and drop some half-assed sanctions. Crimea is important because it is a port in the Black Sea, and Russia ports in the north completely freeze in the winter. When that happens they gotta go through the Black Sea, through Turkey and the Mediterranean Sea.
Back to today there are a couple takes. One is this could mean absolutely nothing just like what happened when they took Crimea. Russia goes in, takes some shyt, and the UN slaps some fukk ass sanctions Sanctions on anything but oil aint gonna do shyt. The thinking is that the West will go after Putin and his cabinents overseas money, which could put pressure on Putin to chill, but that isn't a guarantee.
Another option is UN members decide to goto war with Russia. Now the UN cannot goto war with Russia for one reason: the "cop" of the UN is the Security Council. There are five permanent members: US, France, UK, China (not Taiwan), and Russia. If any of them veto something, it is vetoed period. No way in hell Russia decides against itself, and China has been voting along with Russia since the they took the seat from Taiwan in the 1970s. So any action will be done outside of the UN, also cannot involve NATO because Ukraine is not a member of NATO, and cannot involve the EU because Ukraine isn't in the EU. It'll be each country deciding outside of any international body to take action.
Could that war actually happen? I don't think so. Ukraine isn't worth fighting over. It's really as simple as that. If Ukraine was in NATO, Article 5 (which states an attack on one member is an atttack on all), would've already been invoked and hands and feet would be on Russia right damn now. Since Ukraine isn't in NATO...meh. If Russia really goes out there and shows their ass, it's possible for cyberwarfare to happen. But boots on the ground, planes in the air....I really don't think so. Mutually Assured Destruction is a thing for a reason. Should NATO members openly attack Russia it could lead to the
Shanghai Cooperation Organisation (which is antagonist if you will to NATO and includes the -stans minus Afghanistan, Russia, China, India) popping up and that will 100% be WWIII.
TLDR; nah cause we aint all tryna get fukked up. But it isn't 0% chance.