More than 96% of Crimean voters cast their ballots to break away from Ukraine and rejoin Russia, in a referendum that raises the stakes in the most acute East-West confrontation since the Cold War.

The U.S. and European Union quickly condemned the referendum as illegal. President Barack Obama, speaking by phone with Russian President Vladimir Putin, told him Sunday that the Crimean vote took place "under duress of Russian military intervention" and wouldn't be recognized. The Russian leader responded that the vote...