The Russians literally invaded, the Russians are the ones who are solely responsible for this war.
> ADDEDENDUM: I forgot this, highly relevant for the previously said, quote regarding the NATO (you wrote "strong military alliance like NATO"). NATO's first Secretary General has said that the NATO was created to "keep the Soviet Union out, the Americans in, and the Germans down." [1]
whats totally relevant is the era that, that quote was said in.
NATO was formed in 1949, a mere 5 years after the end of WW2.
Given how Nazi Germany and the USSR had a alliance at the start of WW2, it is wholly unsurprising that NATO, was looking to keep both of them at bay.