Should US Leave NATO?

The purpose of NATO was to to provide collective security for Europe against the Soviet Union. Now the Soviet Union is gone and unlikely to be re-created.

Putin is beating the war drums b/c he views NATO as a threat. Given Russia's quite consistent and long-lived historical fears (some might say "paranoia") about possible invasion, I do not understand why NATO is fiddling around in the Ukraine.

Is there justification for the US remaining in NATO?