Stephen Sackur speaks to former US Deputy Secretary of State William Burns, who worked as a top ranking diplomat for three decades, serving five US presidents. The United States of America is still the most powerful nation on earth but the way it’s perceived by friends and rivals has changed radically in a generation. At the end of the Cold War American supremacy was unchallenged and Washington’s commitment to multilateral global engagement was unquestioned. Are we now in a very different era? Is the US losing its capacity to lead?