Donald Trump loves to say that he will “make America great again”. But Mr Trump’s foreign policy ideas actually amount to a headlong American retreat from “greatness” on the world stage.

The all-but-anointed Republican candidate for the US presidency essentially wants America to resign from the role of global policeman. Any such decision would have profound implications. The world’s security system is based around a series of American “red lines” and alliances. But if the US starts pulling back from its international role, other powers, in particular China and Russia, will move to fill the vacuum.

Mr Trump’s promised America-first trade protectionism could trigger a worldwide recession and undermine the globalised trading system that gives countries an incentive to co-operate rather than compete.