All Conservative leaders since Margaret Thatcher have faced one central problem: how to prevent the party splitting wide apart over Europe. This was the difficulty that pulverised John Major, caused William Hague to go bald, propelled Iain Duncan Smith to the party leadership, and then got him sacked a short time later. Until yesterday, David Cameron’s policy was both sensible and wise: to let sleeping dogs lie. He took heed of Mr Hague’s advice that shifting position on Europe was like moving an unexploded bomb, liable to go off at any moment, across a crowded room. Much better to leave alone. No wonder that the Prime Minister delayed any action for so long, and carried it out with such reluctance. Yesterday, with immense trepidation, the unexploded bomb was moved.