At the beginning of the 20th century, there were 16 empires of varying size and reach. At the end of the century, there was just one: the United States. How did this happen and what role did Britain play in smoothing America’s path to global hegemony?
In the continuing series 'What is...history?' eight historians define political history - an area sometimes regarded as 'narrow', 'elitist' or simply 'dull', but now enjoying a recrudescence.