European Union leaders gather in Rome this week to proclaim their "common future" on the bloc's 60th birthday, despite a wave of crises including Britain's looming exit from the bloc. The 27 leaders will use Saturday's anniversary of the signing of the Treaty of Rome on March 25, 1957, to hail the peace and prosperity they say the project has brought Europe in the aftermath of World War II. But the ghost at the banquet will be British Prime Minister Theresa May, who instead of joining the party in the splendour of a 16th-century palazzo will be in London preparing to trigger the Brexit divorce just four days later. From migration and terrorism to populism and the eurozone debt crisis, Brexit is one of a host of existential challenges clouding the birthday celebrations of a union formed to rebuild Europe from the ashes of war.