Russia fears NATO and its presence in European continent. Ukraine is about to join European Union and NATO alliance. Putin sees that ring of gallows coming close to Russia as ex-Russian-dominated Eastern European countries join NATO alliance. This fear of NATO expansion causing Putin’s Russia the existential crisis. So he reacts to maintain his legacy as the supreme leader of Russia and save the motherland from extinction, even taking a risk of starting the third World War. This is a historical gambling Putin used by the American President Kennedy in October 1962. Khrushchev wanted to place ballistic missiles capable of carrying nuclear war heads in Cuba to protect Communism and its dictator Fidel Castro only fifty miles from American mainland. President Kennedy knew missile batteries around Cuba would create an existential crisis for America. So he ordered to block international sea channel around Cuba and prevent Russian ships from delivering ballistic missiles. Russia pulled out its missiles from Cuba.
Putin wants NATO out of Europe. He supports the idea of European Union will have their own defense pact and military that will eliminate the need for NATO presence in European soil. Putin’s invasion of Ukraine is a tactical move by Putin, which will encourage Europe to have their own defense. This invasion of Ukraine already alerted the European Union members to increase the military expenditure and creation of European defense agency. Now Europe is experiencing the incoming existential threat on the horizon. My guess will be Europe will react to this situation and EU will step towards a defense pact in near future.
Russia pulled its missiles from Cuba and Putin will stop its aggression when NATO is no longer in European theater.