Russian President Vladimir Putin has, in a few months, shattered many European assumptions about the future of their Continent. European governments are still struggling with their response.

"Putin's territorial grab in Crimea" threatens European efforts dating back to the end of World War I to stop territorial change by force, says Fiona Hill, a Russia expert at the Brookings Institution and the author of a biography on the Russian leader. She says 20 years of assumptions about continuing European integration have...