My chain of reasoning:

• if Ukraine loses (or has to enter a very unfavourable agreement), it's not due to Ukraine wanting to lose, but no longer having the means to oppose (after all, Russia is a bigger country and also inherited nearly all the weapons of the USSR, and that was a lot)
• thus, if Ukraine loses, inability or unwillingness of allies to support them is involved
• if subsequently, a victorious Russia recovers economically, Russia may undertake further conquests
• the next conquest could be westward of Ukraine, and a member of NATO --> path A to a really bad war
• however, Russia might be too exhausted to undertake another conquest soon, or might collapse economically, even into revolutionary conditions --> path to uncertain times
• however, there's another conflict waiting to happen: China is systematically training its military for attacking Taiwan, and systematically threatening Taiwan with military maneuvers and political statemements
• it should be noted that China has also had border conflicts with India and Vietnam, and territorial disputes with many more countries, though no claims against their sovereignity
• if Ukraine loses, this implies that Western countries will abandon an ally if pressed hard enough
• subsequently China will consider whether it can press Taiwan hard enough, and I think it will conclude "yes" due to proximity --> path B to a really bad war
• this development might come to a stop though, if Taiwan should surprisingly announce on some year that it has nuclear weapons and conduct a test

Conclusion: I have doubts, but yes, there is a potential for a chain reaction if it's demonstrated that international law does not have enough backers (does not apply if you are big enough).

That's a bold claim, not that I necessarily disagree with it. Are there groups of well respected historians that would agree that it has begun? I could only find a few here and there that think it has started.

It’s possible that U.S. now will tell Ukraine to give up to Russia, what a mess.

World War happens when at least two great powers are on either side, according to League of Nations