You might say it is a match made in heaven. With their taste in gold elevators, the Trump family and the House of Saud were destined to alight at the same penthouse. But the affinity between Donald Trump, US president, and Mohammed bin Salman, Saudi Arabia’s de facto monarch, goes beyond a shared aesthetic for “dictator chic”. It is chiefly transactional. The US-Saudi relationship is the quintessence of Trumpian diplomacy. Its flowering symbolises the decay in the US-led global order.

Mr Trump’s approach to foreign relations is a blend of family and money and a weakness for flattery. Japanese prime minister Shinzo Abe pledged $50m to the Ivanka Trump-inspired Women Entrepreneurs Finance Initiative — the World Bank’s effort to seduce America’s first family. Mr Abe, whose first gift to Mr Trump was a gold-plated golf club, hosted Ms Trump in Tokyo shortly before her father turned up. Six months ago, Saudi Arabia and the United Arab Emirates became the first donors to Ms Trump’s scheme with a $100m grant. Now it is China’s turn to host Mr Trump. Its president, Xi Jinping, approved a flurry of Ivanka Trump trademarks shortly before he first met her father in Mar-a-Lago earlier this year.

Governments the world over are vying to catch the US first family’s eye. But it is the Saudis who perfected the art. It is no coincidence that Riyadh was Mr Trump’s first foreign destination after becoming president. His motive was obvious. The Saudis had agreed to unveil a $110bn arms contract with the US — exactly what Mr Trump was seeking. The fact that it consisted of letters of intent, most of which had been signed years earlier, was beside the point. Saudi Arabia is unlikely to pay for any of the big items. These include US naval vessels that have yet to be built and an anti-missile defence system the Saudis can no longer afford.