Russia would never tolerate the sort of violent intrusion it is now directing against its neighbour. Mr. Putin, evidently fuelled by fantasies of a reborn empire, appears to believe he has a special right to destabilize a sovereign nation based solely on his pursuit of nationalist glory. The world has been here before, but most western leaders understandably thought the horrific legacy of past conflicts European had finally put an end to such mad schemes.

A NATO war with Russia is not an option. President Barack Obama said Thursday Ukraine rebels are “backed, trained, armed, financed by Russia,” but ruled out sending U.S. troops to join the conflict. Ukraine is not a NATO member, though it would like to join. Mr. Obama is correct that the only plausible course is to increase the pain on Moscow to the point that Mr. Putin’s domestic position is substantially weakened. That means imposing a degree of isolation that would turn the country into a pariah state and make clear to ordinary Russians that Mr. Putin’s schemes will bring nothing but economic misery and more secret funerals. Moscow must be made to understand that the cost of its warmongering will be far greater than any potential gains.