Putin has made it clear and warned against NATO involvement re Ukraine that it could mean nuclear war. 

https://www.newsweek.com/nato-moving-missiles-closer-russias-borders-1877490

I recall during the isis times that Putin said if a fight is inevitable, you punch 1st--preemptive attack on the U.S.?

https://www.ft.com/content/d080a162-ee31-3fb6-aa49-92a7e307ec19

Steve M