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.
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.”