IT WAS probably the defining snapshot of Danish politics in 2013: at Nelson Mandela’s memorial service, the country’s prime minister, Helle Thorning-Schmidt, snapped a “selfie” with Barack Obama, the American president, and David Cameron, the British prime minister. The image went immediately viral on the internet.For the three political leaders and their fans it was a moment of harmless fun. For their detractors it was proof of unbecoming frivolity on a solemn occasion.Like any seasoned politician, Ms Thorning-Schmidt took the criticism in her stride and contrived to put a positive spin on the event. The mood was convivial and informal, she told Danish state television afterwards, and a friendly relationship with the American president was only an advantage. “It would have been a problem for Denmark had I sat ramrod straight for three hours and never exchanged a word with President Obama,” she said.Ms Thorning-Schmidt enjoys few lighthearted moments these days: even as she sat down between Messrs Obama and Cameron her phone brought news of yet another government crisis back home: Morten Bodskov, justice minister and…