Mr 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 United States starts pulling back, 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 cooperate rather than compete.
To understand the full implica- tions of Trumpism unleashed, look at three crucial regions - Asia, Europe and the Middle East. It is also important to understand how Mr Trump's deal-making approach to diplomacy would create danger- ous international uncertainty.
In Asia, he would stand US policy on its head. America's approach to the rise of China has been based on economic openness, combined with security alliances designed to balance growing Chinese power.
Mr Trump wants to reverse these priorities. He has talked about imposing swinging tariffs on Chinese goods. But he has also sounded very sceptical about America's two most important treaty alliances in East Asia, with Japan and South Korea.
His priorities probably sound like common sense to many US voters, who regard it as much more important to safeguard American jobs from Chinese competition than to protect Japan and South Korea from potential attack. In reality, his diplomatic "revolution" in Asia would have malign consequences that would swiftly be felt back home.
A Trump-inspired trade war would be seen as a threat to the prosperity and stability of China. If this were combined with a weakening of US security guarantees to its Asian allies, China could well respond aggressively.
The tension in the East and South China seas between China, the US and Japan could escalate into something much more serious.
The implications of Mr Trump's pronouncements on Europe are no less profound. For decades, US policy towards Europe has been based on the twin pillars of Nato and the European Union. But he has abandoned America's traditional support for European integration, suggesting that it would be a good idea if Britain voted to leave the EU.
His scepticism about alliance commitments extends to Nato, which he sees as a bad deal for America. He could take America back to the isolationism of the 1930s, when the US was reluctant to countenance security commitments in Europe. The structural damage he would do to the Western alliance would be compounded by the harm that he would inflict on America's image in Europe. If the Republicans were genuinely to attempt to ban Muslims from entering the US, even temporarily, that would create a huge diplomatic issue with countries such as Britain and France, which could not accept such frank discrimination against millions of their citizens.
The "Muslim ban" would obviously go down even worse in the Middle East, where Trumpism would promise a further deterioration in US regional power.
Many of America's Middle Eastern allies regard President Barack Obama as feckless and unreliable. Mr Trump's scepticism about US alliance commitments, combined with wild promises to grab Middle Eastern oil, are not a formula for the restoration of American regional leadership.
Traditionally, US leaders have made a fetish of "credibility" in international affairs. They believe global security can be guaranteed only if potential adversaries believe America's military commitments are utterly reliable. That is why Mr Obama's failure to enforce his red line against Syria's use of chemical weapons in 2013 was felt to be such a big deal.
Mr Trump, however, has made it clear that he wants America to become more unpredictable. In his writings on business, he has extolled making extravagant demands or promises as an opening bid before compromising.
This might work well in real estate. But it is potentially a formula for disaster in international politics, where America's friends and foes should believe that the US says what it means, and means what it says. Anything else could prompt dangerous miscalculations by US rivals, leading to uncertainty, instability - and, ultimately, war.
THE FINANCIAL TIMES
