Mohammed bin Salman, Saudi Arabia’s crown prince, is a young man in a hurry. So, too, is Jared Kushner, Donald Trump’s son-in-law and senior adviser. Together, they make a dangerous combination. By all accounts, the two men have become firm friends, forging a strong bond melding youth and power. Kushner, 36, made his third visit to Saudi Arabia this year at the end of October. He reportedly talked late into the night with Salman, 32, at the latter’s desert ranch.

Shortly after the meeting, three things happened: Salman began a sweeping purge of wealthy royal rivals; he launched a silent coup in Lebanon; and the Saudi armed forces imposed an aid blockade on Yemeni ports, which (though now partly eased) threatens a humanitarian catastrophe. The White House, supportive of its Saudi friends, made no criticism. Trump tweeted support for the purge. Thanks in part to Kushner, his first foreign trip was to Riyadh, where he was feted by the autocratic regime. He feels a connection.

The strong links between Salman, the de facto Saudi ruler, and the influential Kushner, Trump’s personal overseas troubleshooter and Middle East envoy, are nevertheless a big worry for American diplomats and the Pentagon. Officials told the New York Times they were not briefed on the Salman-Kushner talks. Such secretiveness is apparently the norm. “Jared is a bit of a black hole. There is no sense of the positions he has advocated. We can only guess, based on what he has done and where he has been,” an official said. “The Saudis have been very careful to cultivate him and bring him along.”

How the US under Trump runs its foreign policy is its own business. But when reckless, impulsive and confrontational actions, destabilising the world’s most volatile region, are the result, it’s a problem for everyone. That is what is happening now. As defence minister in 2015, Salman launched the military intervention in Yemen. Its aims were to defeat Shia Muslim Houthi rebels and reduce Iranian influence. It has failed miserably in both. What is has done is turn one of the world’s poorest countries into a killing ground, ravaged by violence, disease and malnutrition.

UN relief organisations warned last week that millions could perish. Save the Children said an estimated 130 Yemeni children are dying every day. More than 50,000 children are believed to have died this year alone – an horrific figure that, coming on the eve of World Children’s Day tomorrow, is deeply shaming.

As Clive Myrie’s graphic BBC television reports last week suggested, the disaster in Yemen is as unacceptable as it is avoidable. But Saudi actions, including alleged crimes against humanity, pass unchallenged by Kushner and a collusional Trump administration.

Visceral Saudi fear of its great regional rival, Iran, lies at the heart of Salman’s many foreign policy miscalculations and mistakes – the cause of growing alarm among western allies, oil buyers and arms suppliers. A furious row has erupted with Germany over Riyadh’s alleged role in forcing the resignation of Lebanon’s prime minister, Sa’ad Hariri, over his refusal to crack down on Iran-backed Hezbollah. Hariri, who his supporters claim was kidnapped by the Saudis, has now taken refuge in Paris.

Circumventing the White House’s silence on Lebanon, Rex Tillerson, the US secretary of state, warned last week the country must not become “a venue for proxy conflicts”. His implied criticism of Riyadh as well as Tehran was significant. France and Germany, mindful of the blockade against Qatar that Salman imposed earlier this year over its links to Tehran, have expressed similar concerns about a widening arc of instability. As Sigmar Gabriel, Germany’s foreign minister, reportedly put it: “Another troublespot is the last thing people in the Middle East need now.”

Salman’s campaign of attrition in Yemen even provoked a mild rebuke from Britain’s foreign office last week, which called for “immediate access for commercial and humanitarian supplies”. This was unusual, given the British government’s habitual subservience to Riyadh. Boris Johnson, the foreign secretary, may have ulterior motives. He is in deep trouble over his mishandling of the case of the wrongly imprisoned British-Iranian woman, Nazanin Zaghari-Ratcliffe. Johnson is due to visit Tehran soon. Perhaps he hopes to curry favour before he arrives.

The sclerotic House of Saud has been viewed historically in the west as a necessary if unattractive force for stability in the Middle East. The biggest criticisms concerned its undemocratic governance, its appalling record on human and civil rights and the ultra-conservative nature of the regime, notably its role in the propagation of Sunni fundamentalism and jihadist ideology. But Salman’s Saudi is increasingly viewed in quite a different light: as an unpredictable, dangerous loose cannon proficient at starting or fuelling conflicts it cannot finish. Its many failings now look less tolerable.

Saudi Arabia is under pressure not just from Iran’s ambitions but also from falling oil revenues, shrinking national wealth and mounting demands for reform. Big changes are undoubtedly required – and in train. Salman’s foolish, headstrong behaviour, sanctioned by his unaccountable pal in the White House, risks it all.