The case against Ukraine joining NATO

Writing in 1997 at age 92, he declared that expanding NATO to the east “ would be the most fateful error of American policy in the entire post-cold-war era.”

“Such a decision,” he went on, “may be expected to inflame the nationalistic, anti-Western and militaristic tendencies in Russian opinion; to have an adverse effect on the development of Russian democracy; to restore the atmosphere of the cold war to East-West relations.”

Note that realism isn't about being realistic but is an ideology/methodology for predicting other nations actions and deciding what actions to take yourself. It involves simplifying nations because realism believes conflict is about about power and ignores ideology, nationalism, politicians personal whims and the will of the people.

Russians might think they are still the Russian bear, but that's long gone now, the world sees them as an annoying mosquito at most.

So Russia stamps its feet to try and meet its imperial goals by dissuading the west from allowing Ukraine into NATO then it can reconstitute its army and have another go.

That’s not possible if Ukraine is in NATO. Which is why Russia screams about NATO expansion and non existent promises for NATO not to expand.

Because it’s very bad for their future ambitions of conquering a bunch of post soviet states in Eastern Europe.