The UK was justified in invading Germany in WWII to protect Europe, even though the UK wasn't originally a threat. Hitler knew that the UK would be an extremely difficult enemy so would have preferred us to stay out of the war. We could have done. We didn't sit by and do nothing because we recognised the threat to Europe was too severe, even if the UK could avoid the same fate.

Some invasions are justified. It's just this Russian invasion isn't one of them.

That's not to say we should be supporting a corrupt Ukrainian regime.