Putin's expansion into Crimea and Ukraine reminds me very much of Hitler's expansion into Austria and Czechoslovakia. Its my firm belief that we're at the "Peace in our time" phase, just before WWIII, and it would be better to nip it in the bud and take action now, rather than 2 years from now, when Putin has had more time to prepare, and he attempts to invade a country that we are obligated by treaty to defend.
Russia is not the USSR of old or Germany in the 30s. It's not a great economic power with an independent industrial base. Right now, Putin is throwing in all the conventional forces he can spare, but Ukraine – a smaller and poorer country - has managed to stop the attack with some material support from the West.
Meanwhile, the rest of the world has not been idle. Europe is increasing military spending significantly. Unprecedented sanctions are in place, and their purpose is to destroy Russian economy and cut their access to many key technologies. If the sanctions remain in place, a large part of the Russian losses in Ukraine will be permanent.