Just hours after engineering the biggest gamble in US foreign policy since Richard Nixon went to China, John Kerry, the Vietnam veteran turned chief US diplomat, explained what drove him to a deal with an Iranian regime that routinely shouts “Death to America”.

“When I left college, I went to war. And I learnt in war the price that is paid when diplomacy fails,” said the former five-term senator who narrowly lost the 2004 presidential race to George W Bush. “I made a decision that if I ever was lucky enough to be in a position to make a difference, I would try to do so.” 

While President Barack Obama will forever be associated with one of the most controversial agreements in US foreign policy, Mr Kerry made it happen. Speaking in the 19th-century Palais Coburg in Vienna, he joked that the day was historic because it was the “first time in six weeks that I’ve worn a pair of shoes” — after his recovery from a cycling accident in France. But in truth the deal to prevent Iran from obtaining a nuclear weapon was a legacy-defining victory for a man whose long career had lacked a crowning achievement.