On 19 June 1815, after the battle of Waterloo, the Duke of Wellington declared that ‘nothing except a battle lost can be half so melancholy as a battle won’. Two hundred years later, David Cameron or Ed Miliband might feel the same way as they sit in Downing Street. Any elation over victory will be quickly overshadowed by the thought of troubles to come — in all likelihood insurmountable troubles for either man.