Any NATO country can join forces with Ukraine and help them defend ukrainian territory. Do you think Russia would dare to strike NATO territory in retaliation? I dont.A country cannot join NATO if it is undergoing armed conflict. Ukraine would only be allowed to join if it either defeats Russia or they agree a peace settlement. That is why Ukraine could not join NATO since 2014 because of the armed conflict in the Donbas. It's in Russia's interest to keep this conflict simmering, so that Ukraine can not join NATO.