Throughout the Middle East’s modern history, a constant remained – the US held a prominent stake and would throw its weight around to protect its interests and allies. The maxim held true as ideologies rose and fell, Gulf monarchies, Israel, and Arab nationalist police states took root – and war and insurrection periodically raged.

But it ended during Donald Trump’s third year, a time when an isolationist, unworldly president began to see regional interests through a much narrower lens. The effect has been profound and 2020 will continue the process of recalibration by traditional friends of the US without a country whose clout they used to defer to and whose agenda they could more or less understand.

Allies such as Saudi Arabia bought weapons and patronage from Washington, which they banked for when it might matter one day. That moment came for Riyadh in September when a drone and missile attack launched from Iran destroyed half of the country’s oil production capacity, following a summer of Iranian challenges to Gulf shipping.

Saudi leaders believed the US would invoke an unofficial defence pact and strike back on their behalf but Trump backed down – Iran having successfully called his bluff – and an embarrassed Kingdom was left exposed.

The response took only days: Saudi Arabia sought a detente with Iran, via Tehran’s ally Iraq. Both sides, polarised by Yemen, Syria and more ancient rivalries have been converging ever since.

Across the region, there is a clear sense that the guard has changed. “If the Americans want to make it all transactional, then fine,” said one Iraqi leader. “We will buy their toys, and sell their ideals. We never liked them anyway.”

Less than one month later, Trump made the surprise decision to abandon Kurdish forces, with whom US troops had partnered to fight Isis. The move came as Turkey was poised to invade the Kurdish north of Syria, sacrificing an ally who had safeguarded Europe and the US over five gruelling years, to a foe it could not match in a conventional war.

The Saudi decision had startled Washington’s friends, but the betrayal of the Kurds truly shocked them. Even Israel, a friend of the Kurds and a beneficiary of Trump in the region, was rattled. If a US president could do such a thing on a whim, could he also overturn a bedrock pillar of foreign policy carried over by every US leader for decades?

The answer was no. To do so would enrage his domestic base and kill his hopes of a second term. Nevertheless, both decisions left Israel and other allies much shakier than they had expected under a leader who had touted the strength of bilateral ties throughout his first years in office.

As Trump disengages, convinced that there is little to gain by US strategic engagement in the broader region and that his predecessors had the “light on the hill notion” wrong, the vacuum created by the US departure has been quickly filled. Russian flags now fly over US bases vacated in haste in Syria. Vladimir Putin has been on a state visit to Riyadh. His aides rotate through Beirut, Baghdad and Erbil, having already staked a claim in Damascus.

In mid-November, a Syrian tycoon with influential Kremlin ties spoke of the change in mood in the Middle East. “All my Saudi friends are saying to me: ‘Those contacts and plans you’ve been telling us about for 16 years, implement them all now. You (Russians) guys are the future. The Americans have left.’”

It has also weakened US friends and allies in real terms. The one regional state the US had remained focused on, Iran, emerged emboldened from Trump’s climbdowns. “Maximum pressure” had been the term used to describe a sanctions regime on Iran reimposed and tightened by Trump, designed to cripple its economy and force it to renegotiate. The squeeze was working, but, paradoxically, having poked the bear and found it sleeping, Iran now had more room to manoeuvre regionally.

The impact of the US failing to respond to an attack on Saudi oil facilities was that an act of war on a US ally had gone unpunished, and that ally was now willing to talk with the country that Washington had been determined to bring to its knees. Ways around the sanctions may follow.

But in the meantime, Iran is, for the first time facing a three-pronged threat to its reach and influence, with protesters challenging governments at home, as well as in Iraq and in Lebanon. The risk of security collapse is real and growing, and it remains unclear if Trump is prepared to own the consequences of actions that the US helped set in motion.