When the US embassy in Baghdad was mobbed on the last day of 2019, a rattled Donald Trump turned to his most hawkish aide, Mike Pompeo, and finally agreed to the extreme measure the secretary of state had long advocated: the assassination of Qassem Suleimani.

The drone strike on the Iranian general a week ago may well turn out to be one of the most consequential decisions of the Trump presidency. It represented a significant victory for Pompeo, reflecting his ascendancy in foreign policy and national security spheres.

It also highlighted the dwindling of restraining influences in an administration for which confrontation with Iran has become, in the words of one European diplomat, “the one central organising principle that everybody agrees on”.

Rob Malley, a former senior official in the Obama national security council and now president of the International Crisis Group, said: “What is clear is that Iran is a fixation and obsession for this administration. And it certainly seems to be a fixation of Secretary Pompeo’s.”

“Pompeo is an Iran hawk, and is more powerful than anyone else in the inner circle,” said Kirsten Fontenrose, former director for the Gulf in Trump’s national security council (NSC), who added that his clout far outweighed the relative newcomers, the defence secretary, Mark Esper, and national security adviser, Robert O’Brien.

“Pompeo has the credibility of having been CIA director. [Gina] Haspel is CIA director now, but Pompeo is the president’s whisperer. He can say what the language [in the president’s intelligence briefs] means, and the president listens to him very closely. He has an outsize influence.”

According to multiple reports, the killing of Suleimani had been presented to Trump as an option several days earlier, when a 27 December attack on an Iraqi military base by an Iranian-backed Shia militia killed an American contractor. On that occasion the president ignored Pompeo’s lobbying and chosen a less escalatory reprisal: airstrikes against the militia, Kata’ib Hezbollah.

But that led to a counter-punch: Shia militiamen and their supporters rushed into Baghdad’s fortified diplomatic quarter, the Green Zone, with no resistance from Iraqi checkpoints and overran the gates of the US embassy compound.

The rioters failed to breach the embassy building itself, Iraqi forces eventually arrived on the scene and the militiamen dispersed. There were no US injuries. But the incident unnerved the president. According to an official quoted by CNN, it did not just remind Trump of the assault on the US consulate in Benghazi in 2012 when four Americans were killed, but also the 1979 seizure of the US embassy in Tehran after the Iranian revolution, in which 52 US diplomats and staff were taken hostage.

The political lesson was clear. The Iran hostage crisis proved fatal for Jimmy Carter’s presidency, while the Benghazi attack was used for years to lambast the secretary of state at the time, Hillary Clinton, most of all by then congressman Mike Pompeo.

Against that backdrop, Pompeo was pushing at an open door last week when he warned that of the risks of the showing weakness in the Oval Office.

It also helped Pompeo’s cause that the decision was being made while Trump was on holiday at his Florida resort, Mar-a-Lago, without a full complement of national security staff around him.

John Gans, a former Pentagon speechwriter and author of a new book White House Warriors: How the National Security Council Transformed the American Way of War, said those circumstances allowed Pompeo to score “an end run” around the policymaking process.

“It was very clear that this thing wasn’t staffed through,” Gans said. “And Pompeo was able to say hey: you didn’t choose this option because people thought it was too aggressive earlier in the week, and then he sold him on it. Trump launches, and here we are.”

Other former officials and analysts argue that Pompeo would have eventually got his way in any circumstances because of the departure over the past three years of voices of caution. According to security analyst Peter Bergen in a recently published book, Trump and his Generals, the former defence secretary James Mattis refused to provide the White House aggressive options on Iran.

According to a diplomatic source, the advice of the former chairman of the joint chiefs Gen Joseph Dunford was a key factor in Trump’s last-minute decision to call off airstrikes against Iran in June. Their replacements, Esper, a former defence industry lobbyist, and Gen Mark Milley, have not played such a restraining role.

At the NSC, traditionally where decisions are debated, there is also no one with the experience and authority to second-guess Pompeo. O’Brien is a lawyer with little national security experience. His deputy, Victoria Coates, who is also senior director for the Middle East, is trained as an art historian, and gained her foreign policy experience as an adviser to Senator Ted Cruz, another hardliner on Iran.

Below Coates, a former special forces officer, Rob Greenway, has been given an expanded role to manage policy across the Middle East.

“He has taken over the whole region, and he has spread the lens of Iran over the whole region,” said a former Trump official. “He has been given the job of coordinating an economic war against Iran, and for him, it is all about beating Iran in all these other theatres.”

According to Fontenrose, a major Yemen reconstruction initiative was sacrificed to the exclusive focus on Iran.

“Part of that focus is appropriate,” she said. “But unfortunately the full vision has been lost. There is no nuance, no light and shade.”

Some analysts, however, pointed to some recent evolution from a purely Iran-centric view of the Middle East. In a departure last month, the state department Iran envoy, Brian Hook, acknowledged that the Houthi movement in Yemen acted independently from Tehran, and was not just a proxy.

“The administration’s policy focus extends far beyond the Iran file,” a senior administration official said. “Other major priority areas include the Middle East Strategic Alliance, Libya, Israel, Yemen, Syria, Lebanon and Iraq, to name a few. In fact, Victoria Coates is currently overseas meeting with Libyans to help resolve the conflict in their country, which speaks to our versatility.”