Donald Trump in a second term would likely install loyalists in key positions in the Pentagon, US State Department and CIA whose primary allegiance would be to him, allowing him more freedom than in his first presidency to enact isolationist policies and whims, nearly 20 current and former aides and diplomats said.
The result would enable Mr Trump to make sweeping changes to the US stance on issues ranging from the Ukraine war to trade with China, as well as to the federal institutions that implement - and sometimes constrain - foreign policy, the aides and diplomats said.
During his 2017-2021 term, Mr Trump struggled to impose his sometimes impulsive and erratic vision on the US national security establishment.
He often voiced frustration at top officials who slow-walked, shelved, or talked him out of some of his schemes. Former US defence secretary Mark Esper said in his memoir that he twice raised objections to Mr Trump's suggestion of missile strikes on drug cartels in Mexico, the US' biggest trade partner. The former president has not commented.
"President Trump came to realise that personnel is policy," said Robert O'Brien, Mr Trump's fourth and final national security adviser. "At the outset of his administration, there were a lot of people that were interested in implementing their own policies, not the president's policies."
Loyalists
Having more loyalists in place would allow Mr Trump to advance his foreign policy priorities faster and more efficiently than he was able to when previously in office, the current and former aides said. Among his proposals on the campaign trail this year, Mr Trump has said he would deploy US Special Forces against the Mexican cartels - something unlikely to get the blessing of the Mexican government.
If he returns to power again, Mr Trump would waste little time cutting defence aid to Europe and further shrinking economic ties with China, the aides said.
Mr O'Brien, who remains one of Mr Trump's top foreign policy advisers and speaks to him regularly, said imposing trade tariffs on Nato countries if they did not meet their commitments to spend at least 2 per cent of their gross domestic product on defence would likely be among the policies on the table during a second Trump term.
The Trump campaign declined to comment for this article.
Unlike in the lead-up to his 2016 election, Mr Trump has cultivated a stable of people with whom he speaks regularly, and who have significant foreign policy experience and his personal trust, according to four people who converse with him.
Those advisers include John Ratcliffe, Trump's last director of National Intelligence, former US ambassador to Germany Richard Grenell, and Kash Patel, a former Trump staffer who held several positions in the intelligence and defence communities.
None of those people responded to interview requests.
While the specific policies of these informal advisers vary to some degree, most have been vocal defenders of Mr Trump since he left office and have expressed concerns that America is paying too much to support both Nato and Ukraine.
'Doomsday option'
Mr Trump has a commanding lead in the Republican presidential nomination race. If he becomes the Republican nominee and then defeats Democratic president Joe Biden next November, the world will likely see a much more emboldened Trump, more knowledgeable about how to wield power, both at home and abroad, the current and former aides said.
That prospect has foreign capitals scrambling for information on how a second Trump term would look. Mr Trump himself has offered few clues about what kind of foreign policy he would pursue next time around, beyond broad claims like ending the Ukraine war in 24 hours.
Eight European diplomats interviewed by Reuters said there were doubts about whether Mr Trump would honour Washington's commitment to defend Nato allies and acute fears he would cut off aid to Ukraine amid its war with Russia.
One Northern European diplomat in Washington, who spoke on condition of anonymity because of the sensitivity of the issue, said he and his colleagues had kept talking to Trump aides even after the former president left the White House in 2021.
"The story from there was, 'We were not prepared (to govern), and next time it has to be different,'" the diplomat said. "When they got into the Oval Office in 2017, they didn't have any idea what the hell to do with it. But this won't happen again."
The diplomat, whose country is a Nato member, and one other diplomat in Washington said their missions have outlined in diplomatic cables to their home capitals a possible "doomsday option".
In that hypothetical scenario, one of multiple post-election hypotheses these diplomats say they have described in cables, Mr Trump makes good on pledges to dismantle elements of the bureaucracy and pursue political enemies to such a degree that America's system of checks and balances is weakened.
"You have to explain to your capital. 'Things might go rather well: the US keeps on rehabilitating herself' (if Biden is re-elected)," said the diplomat, describing his mission's view of American politics. "Then you have Trump, a mild version: a repetition of his first term with some aggressive overtones. And then you have the doomsday option."
Retreat from globalism
Michael Mulroy, the deputy assistant secretary of defence for the Middle East under Mr Trump, said the former president would likely appoint individuals who subscribed to his isolationist brand of foreign policy and were unlikely to confront him.
All US presidents have the power to name political appointees to the most senior jobs in the federal bureaucracy, including the State Department, Pentagon and the CIA.
"I think it will be based primarily on loyalty to President Trump," Mr Mulroy said, "a firm belief in the kind of foreign policy that he believes in, which is much more focused on the United States, much less on a kind of globalist (policy)."
Mr Trump clashed with his own appointees at the Pentagon on a number of issues in his first term, from a ban on transgender service members that he supported to his 2018 decision to pull US troops from Syria.
When his first defence secretary, Jim Mattis, resigned in 2018, the former four-star general stated he had significant policy differences with Mr Trump. While Mr Mattis did not explicitly lay them out, he stressed in his resignation letter the need to maintain an ironclad bond with Nato and other allies, while keeping enemies, like Russia, at arms-length.
Ed McMullen, Mr Trump's former ambassador to Switzerland and now a campaign fund-raiser who is in contact with the former president, stressed that most foreign service personnel he knew served the president faithfully.
But, he said, Mr Trump was aware of the need to avoid choosing disloyal or disobedient officials for top foreign policy posts in a second term.
"The president is very conscious that competency and loyalty are critical to the success of the (next) administration," he said.
Agenda47
Outside of Mr Trump's top circle of advisers, a potential Trump administration plans to root out actors at lower levels of the national security community perceived to be "rogue," according to Agenda47, his campaign's official policy site.
Such a step would have little precedent in the United States, which has a non-partisan bureaucracy that serves whichever administration is in office.
Mr Trump has said he plans to reinstate an executive order he issued in the final months of his first term, which was never fully implemented, that would allow him to more easily dismiss civil servants. In a little-reported document published on Agenda47 earlier this year,
Mr Trump said he would establish a "Truth and Reconciliation Commission," which would, among other functions, publish documents related to "Deep State" abuses of power. He would also create a separate "auditing" body meant to monitor intelligence gathering in real time. "The State Department, Pentagon, and National Security Establishment will be a very different place by the end of my administration," Mr Trump said in a policy video earlier this year.
Nato pullout? New trade war
During a second term, Mr Trump has pledged to end China's most favoured trading nation status - a standing that generally lowers trade barriers between countries - and to push Europeans to increase their defence spending.
Whether Mr Trump will continue vital US support for Ukraine in its war with Russia is of particular importance to European diplomats in Washington trying to prepare, as is his continued commitment to Nato.
"There are rumours that he wants to take the US away from Nato or withdraw from Europe. Of course it sounds worrying but ... we are not in a panic," said a diplomat from one Baltic state.
Despite worries about the future of Nato, several diplomats interviewed for this article said pressure from Mr Trump during his first term did lead to increased defence spending.
John Bolton, Mr Trump's third national security adviser who has since become a vocal critic of the former president, told Reuters he believed Mr Trump would withdraw from Nato.
Such a decision would be earth-shaking for European nations that have depended on the alliance's collective security guarantee for nearly 75 years.
Three other former Trump administration officials, two of whom are still in contact with him, played down that possibility, with one saying it would likely not be worth the domestic political blowback.
At least one diplomat in Washington, Finnish ambassador Mikko Hautala, has spoken to Mr Trump directly more than once, according to two people with knowledge of the interactions, which were first reported by The New York Times.
Those discussions centred on the Nato accession process for Finland. Mr Hautala wanted to make sure Mr Trump had accurate information about what Finland brings to the alliance and how Finland joining benefits the US, one of the people said.
