The outgoing French ambassador to the US has compared the Trump administration to the court of King Louis XIV, filled with courtiers trying to interpret the caprices of a “whimsical, unpredictable, uninformed” leader.

Gérard Araud, who retires on Friday after a 37-year career that included some of the top jobs in French diplomacy, said Donald Trump’s unpredictability and his single-minded transactional interpretation of US interests was leaving the administration isolated on the world stage.

“When they say ‘America first’, it’s America alone,” Araud said in an interview with the Guardian. “Basically, this president and this administration don’t have allies, don’t have friends. It’s really [about] bilateral relationships on the basis of the balance of power and the defence of narrow American interest.”

He cautioned the UK against expecting any special treatment from Washington in post-Brexit trade talks, predicting that the administration would force London to accept US imports on US terms, such as looser standards for genetically modified organisms (GMOs).

“They [the Trump administration] are not thinking in terms of multilateral cooperation first. And secondly, they don’t have any affection towards the Europeans. They treat Europeans the way they treat the Chinese,” Araud said. “And when the British come for a free-trade agreement, there will be blood on the walls and it will be British blood. It will be GMOs breakfast, lunch and dinner.”

Araud has been France’s ambassador to Washington since 2014, and before that was the country’s envoy to the UN and to Israel, and the foreign ministry’s director for strategic affairs, security and disarmament.

In Washington he stood out among the diplomatic corps in part because he lived with his partner, the photographer Pascal Blondeau, at a time when the administration is increasingly inhabited by conservative evangelical Christians hostile to gay marriage.

The couple threw spectacular parties at the chateau-like French residence. The invitation to their last winter bash featured them posing with a stuffed white tiger, and guests were welcomed on the night by Blondeau reclining in a lumberjack shirt on a large illuminated Christmas tree tipped on its side in the main hall. There was also a stuffed polar bear and ballet dancers.

Araud was also unusual on the diplomatic circuit for his blunt and acerbic language – in person and on Twitter – in defence of multilateral diplomacy, liberalism and international law at a time when they have been under siege.

At 2am on the night Trump was elected, the ambassador tweeted: “It is the end of an era, the era of neoliberalism. We don’t yet know what will succeed it … After Brexit and this election, anything is possible. A world is collapsing before our eyes. Vertigo.”

He deleted the tweet minutes later, even before the anxious calls started coming in from the Quai d’Orsay in Paris, but few now would dispute that this moment in the early hours of 9 November 2016 marked a cliff edge in modern history. And Araud himself could not have foreseen the volatility that was to engulf US foreign policy.

Facebook Twitter Pinterest Donald Trump and the French president, Emmanuel Macron, in 2017. Photograph: Evan Vucci/AP

Araud spoke to the Guardian before Trump tweeted unsolicited, impractical and potentially disastrous advice for putting out the Notre Dame fire, but after a period of whiplash on other issues. These have been difficult times for diplomatic missions tasked by their capitals to predict US policy.

On the issue of North Korea’s nuclear arsenal, for example, Trump has gone from threatening “fire and fury” against the Pyongyang regime, to meeting Kim Jong-un in Singapore and declaring himself to be “in love” with the dictator. He said then he was in “no hurry” for disarmament, but at a second summit in Hanoi in February he demanded full denuclearisation in advance of any sanctions relief, before cancelling sanctions imposed by his own administration in March because “President Trump likes Chairman Kim” and declaring himself once more open to a “step-by-step” approach.

“It’s like [trying] to analyse the court of Louis XIV,” Araud said. “You have an old king, a bit whimsical, unpredictable, uninformed, but he wants to be the one deciding.”

Like the Sun King who dominated France in the 17th and 18th centuries, Trump “doesn’t want to appear under any influence and he wants to show it”, Araud said.

He portrayed the current situation as the opposite extreme of the meticulous though sometimes ponderous decision-making process pursued by the previous administration.

“Obama was the ultimate bureaucrat: you know every night he was going to bed with 60 pages and in the morning they were coming back all annotated by the president,” he said. For decisions such as the troop surge in Afghanistan, there were months of meetings between the relevant government departments.

Now that inter-agency process is largely dead, killed off and replaced by John Bolton, the ultra-hawkish national security adviser, while other centres of power in the state department and Pentagon are withering, weakened by multiple unfilled senior positions, and top officials serving in acting capacity only, without Senate confirmation.

“Actually, we don’t have interlocutors,” Araud said. “[When] we have people to talk to, they are acting, so they don’t have real authority or access. Basically, the consequence is that there is only one centre of power: the White House.

“Bolton is really very competent. He is very knowledgeable. He has been around for 40 years. At the same time you have to understand that he doesn’t control this president, because this president is uncontrollable.”

Araud said the tweeted cancellation of North Korean sanctions, and the exclusion of Bolton from key meetings and meals at the Hanoi summit, were designed to “humiliate” Bolton and demonstrate that the president “is the master and the bureaucrats are nothing”.

Bolton’s unilateralism is nonetheless ascendant for the time being, with the US withdrawal from the Intermediate-range Nuclear Forces (INF) arms control treaty, and the recognition of Israel’s annexation of the Golan Heights, which Araud described “as one more nail in the coffin of international law”.

He said Bolton’s view was that international law was mere convention, with “no gendarme and no judge” to enforce it. “But it’s a sort of a fragile wall or dam against barbarians,” he said.

Araud expressed anxiety about the implications of the administration’s maximum-pressure campaign against Iran, championed by Bolton and the secretary of state, Mike Pompeo.

“[The policy is] increasing the pressure until the Iranians surrender, or in some quarters they believe until the regime collapses,” Araud said. “But if the regime collapses, what happens? And to this question the Americans are unable to answer.”

Leaving his post, Araud said he was more convinced than ever that the fear expressed in his deleted election-night tweet had been borne out by events.

“I had a lot of trouble with my own capital. Unfortunately, to be right early is to be wrong,” he observed. “I had deleted it after two minutes but the sin had been committed. But looking at it in retrospect, of course I was right.

“My world, our world of certainties, really was collapsing and we were facing a real, substantial, dangerous crisis, which could basically really overwhelm my own country,” Araud said. “I believe we are entering a new era. I just don’t know what this era will be.”