Try it and Kiev is Russian in 2 months.
... and then WW III breaks out and it's game over for everyone.
GG.
Did the WW3 broke out after Russia had a war with Georgia in 2008? No. Did any of the NATO boyfriends came to the rescue of Georgia? No. The same will happen now. None of the NATO nations want to have a direct confrontation with Russia. They are not stupid enough to sacrifice their soldier's lives to defend a third world nation such as Ukraine.
The worst they can do is to impose sanctions on Russia and that's not going to stop Putin from doing what he wants to do, The USA will not take stand with Trump in power and neither would the rest take part, with Russia's problems with Ukraine, i doubt Ukraine will ever be granted membership into NATO.