It is always easy to criticize. What do you think should happen? Just yield Ukraine to Putin? Did not work with Germany and Elsass-Lothringen, did not work with Russia and the Crimea, won’t work now.
What happens if you give in to a dictator’s demands isn’t that he is satisfied and stops. Instead he will realize that he got away with it, then try something bigger next time. There are countless examples of this in history.