What will it take to end the war Vladimir Putin has initiated in Ukraine? In military terms, Russia now seems intent on a grim campaign of attrition in the east and south - a strategy which is already taking a terrible human toll. Could economic isolation inflict enough pain to force the Kremlin to reconsider? Stephen Sackur speaks to the exiled Russian economist Sergei Guriev. Is Moscow outmanoeuvring the west when it comes to sanctions?