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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.

Mr Cameron took careful note. In opposition he did his best to avert trouble, and was almost excessively careful during the early period of government. Europe was the easiest part of the Coalition Agreement to negotiate, Mr Cameron having already abandoned his “cast-iron” guarantee of a referendum on the Lisbon Treaty....