At the outbreak of the First World War, investors sold dollars and bought sterling. It ought to have been the reverse, since Britain had declared war and the US was standing apart. But crowds rush to safety in a crisis – and sterling was then the reserve currency. Little did they guess that sterling would be undone by war debt.

Markets reacted the same way after the 2008 Lehman Brothers collapse. Although the crisis began on Wall Street, the dollar rose; America was rewarded for its sins. But profligacy cannot go on indefinitely. By his actions, Donald Trump is bringing forward the dollar's reckoning.

Financial Times