Putin probably knows he might have a limited time to act because the U.S. might get a hawkish President (Hillary Clinton, any Republican but Rand Paul) or Germany might re-militarize.

In this conflict Germany wants negotiate a deal that accepts a Russian Crimea in exchange for Ukrainian control of the rest of Russia and the US oppose such a deal. Germany has a stronger interest in keeping the Russian gas pipelines active than it has in whether or not Russia controls Crimea.

Defending an EU country like Finland would have a completely different priority. I doubt that Germany would have a huge problem with Russia taking over Belarus either. Minsk is anyway badly governed.

Bismark might be a better comparison then 1938-39.

I don't think the key problem is US unwillingness to fight but the unwillingness to engage in diplomacy make a deal that gives Russia officially Crimea in exchange for keeping the rest of Ukraine under the control of Kiev.