Vladimir Putin looks likely to go down in history as the Russian leader who won back Crimea, but he is fighting to avoid also being remembered as the man who let Ukraine escape from Moscow's sphere of influence.

The next six weeks will be decisive in that battle as Ukraine prepares to elect a president on May 25, agree a new constitution and prevent pro-Russian separatists in the east breaking away.

Reclaiming Crimea 60 years after it was gifted to Ukraine by Soviet leader Nikita Khrushchev, apparently on a whim, has been a domestic triumph for Putin. But losing the ability to exert influence over Ukraine would be a high price to pay for that and might, with time, seriously detract from the achievement.