When David Cameron became Tory leader seven years ago, William Hague is said to have delivered a stern warning on the subject of Europe. Stay well away, advised Mr. Hague, who knew from brutal personal experience as Conservative leader the damage it was capable of doing. Mr. Hague added that Europe should be regarded as a bomb that could never be defused, yet might well go off at any moment. The wisest course was to leave well alone and hope for the best. … It has suddenly become very clear that, seven years after the Foreign Secretary delivered his warning,...