Whatever his myriad faults and foibles, Boris Johnson has the one essential quality that Napoleon demanded of his generals: luck. A few weeks ago, the Prime Minister was on the point of being dethroned by his own MPs. Today, thanks to two men, Vladimir Putin and Rishi Sunak, he strides the stage again, basking in the praise of president Volodymyr Zelenskyy as Ukraine’s most doughty and effective international defender.