Also, to clarify one point: No one is a candidate for NATO who is currently engaged in hostilities. While Ukraine was in a state of war against Russian support

Also, to clarify one point: No one is a candidate for NATO who is currently engaged in hostilities. While Ukraine was in a state of war against Russian supported forces in Donbass and Crimea, it was ineligible to even apply. It may have put the goal of joining NATO in its constitution, but it was a non-starter until that conflict was resolved. BTW, Russia has shared borders with multiple NATO countries, starting with Norway in 1949 when NATO was founded, and the Baltics since 2004. A neighbouring country's membership in an alliance is not a casus belli.