As the president of the Center for Strategic and Budgetary Assessments, Andrew Krepinevich’s job is to think about the security challenges of the future. He and his organisation have been working on a strategy to present to the next US president’s transition team.

Mr Krepinevich, who spent 21 years in the US army, has been an adviser to many senior officials in the Pentagon and has also guided US allies on strategy. As he sits in his corner office overlooking Washington’s K Street, he identifies four overlapping security questions for the US and its allies in the 21st century.