Defend them against who or what?
The ussr is no more and apparently russia cannot conquer Ukraine.
Unable to shed an imperial past and a taste for autocratic rule, Russia is aggressive and thuggish by nature, which makes her a menace to her immediate neighbors and to American allies in the region. Without US and NATO help, Russia would consume Ukraine. Russian statements indicate that they regard the entire former Warsaw Pact area as naturally subject to Russian control or influence. That puts Eastern and Central Europe to the Elbe river in Germany on the Russia menu.