As president, Donald Trump selectively revealed highly classified information to attack his adversaries, gain political advantage and to impress or intimidate foreign governments, in some cases jeopardizing US intelligence capabilities.
As an ex-president, there's every reason to worry he will do the same, thus posing a unique national security dilemma for the Biden administration, current and former officials and analysts said.
All presidents exit the office with valuable national secrets in their heads, including the procedures for launching nuclear weapons, intelligence-gathering capabilities - including assets deep inside foreign governments - and the development of new and advanced weapon systems.
But no new president has ever had to fear that his predecessor might expose the nation's secrets as President-elect Joe Biden must with Mr Trump, current and former officials said.
Not only does Mr Trump have a history of disclosures, he checks the boxes of a classic counterintelligence risk: He is deeply in debt and angry at the US government, particularly what he describes as the "deep state" conspiracy that he believes tried to stop him from winning the White House in 2016 and what he falsely claims is an illegal effort to rob him of re-election.
"Anyone who is disgruntled, dissatisfied or aggrieved is a risk of disclosing classified information, whether as a current or former officeholder. Trump certainly fits that profile," said David Priess, a former CIA officer and author of The President's Book of Secrets, a history of the top-secret intelligence briefings that presidents and their staff receive while in office.
The White House did not respond to a request for comment.
As president, Mr Trump has access to all classified information in the government and the authority to declassify and share any of it, for any reason. After he leaves office, he still will have access to the classified records of his administration. But the legal ability to disclose them disappears once Mr Biden is sworn in January.
Many concerned experts were quick to note that Mr Trump reportedly paid scant attention during his presidential intelligence briefings and has never evinced a clear understanding of how the national security apparatus works. His ignorance may be the best counterweight to the risk he poses.
"A knowledgeable and informed president with Trump's personality characteristics, including lack of self-discipline, would be a disaster. The only saving grace here is that he hasn't been paying attention," said Jack Goldsmith, who ran the Office of Legal Counsel at the Justice Department in the George W. Bush administration and is the co-author of After Trump: Reconstructing the Presidency.
"He probably doesn't know much about collection details. But he will have bits and pieces," said retired Gen Peter B Zwack, who served as a military intelligence officer and was the senior US defence attache to Russia from 2012 to 2014.
The chances are low that Mr Trump knows the fine details of intelligence, such as the name of a spy or where an intelligence agency may have planted a surveillance device. But he almost certainly knows significant facts about the process of gathering intelligence that would be valuable to adversaries.
"The president is going to run into and possibly absorb a lot of the capacity and capabilities that you have in intelligence," said John Fitzpatrick, a former intelligence officer and expert on the security systems used to protect classified information, including after a president leaves office. The kinds of information Mr Trump is likely to know, Mr Fitzpatrick said, includes special military capabilities, details about cyber weapons and espionage, the kinds of satellites the United States uses and the parameters of any covert actions that, as president, only Mr Trump had the power to authorize.
He also knows the information that came from US spies and collection platforms, which could expose sources even if he didn't know precisely how the information was obtained. In a now infamous Oval Office meeting in 2017, Mr Trump told Russia's foreign minister and ambassador to the United States about highly classified information the United States had received from an ally about Islamic State threats to aviation, which jeopardized the source, according to people familiar with the incident.
By bragging about intelligence capabilities, Mr Trump put them at risk. And he has been similarly careless when trying to intimidate adversaries. In August 2019, he tweeted a detailed aerial image of an Iranian launchpad. Such photos are among the most highly guarded pieces of intelligence because they can reveal precise details about technical spying capabilities.
Using publicly available records, Internet sleuths were able to determine which satellite took the image and identify its orbit based on the image Mr Trump disclosed.
Experts worry that Mr Trump's braggadocio may lead him to spill secrets at a rally or in a tete-a-tete with a foreign adversary. One former official imagine Mr Trump boasting about the technical features of Air Force One, or where the United States had dispatched spy drones.
Mr Trump has also demonstrated a willingness to declassify information for political advantage, pushing his senior officials to reveal documents from the 2016 probe of Russian election interference and possible links to Mr Trump's campaign.
Last month, Director of National Intelligence John Ratcliffe, a Trump loyalist, made public a set of handwritten notes and a referral to the FBI concerning intelligence that the United States had obtained on Russia, and its belief that Hillary Clinton's presidential campaign would try to tie the hacking and leaking of Democratic Party emails to Russia to deflect from the controversy around Ms Clinton's use of a private email server.
Those declassified documents were heavily redacted. But according to people familiar with their contents, they may have revealed enough information to point the Russian government to a valuable source of intelligence the United States has, and is now at risk of losing.
Experts agreed that the biggest risk Mr Trump poses out of office is the clumsy release of information. But they didn't rule out that he might trade on secrets, perhaps in exchange for favours, to ingratiate himself with prospective clients in foreign countries or to get back at his perceived enemies. When he leaves office, Mr Trump will be facing a crushing amount of debt, including hundreds of millions of dollars in loans that he has personally guaranteed.
"People with significant debt are always of grave concern to security professionals," said Larry Pfeiffer, a veteran intelligence officer and former chief of staff to CIA Director Michael V Hayden. "The human condition is a frail one. And people in dire situations make dire decisions. Many of the individuals who've committed espionage against our country are people who are financially vulnerable."
As a practical matter, there's little that the Biden administration can do to stop Mr Trump from blurting out national secrets. Former presidents do not sign nondisclosure agreements when they leave office. They have a right to access information from their administration, including classified records, said Mr Fitzpatrick, who served as the director of the Information Security Oversight Office at the National Archives and Records Administration, which houses former presidents' records.
They're expected to safeguard information, as they did while in office. "But outside the confines of the Presidential Records Act, there is no boundary except the president's behavior," he said.
A President Biden could refuse to give Mr Trump any intelligence briefings, which ex-presidents have received before meeting with foreign leaders or embarking on diplomatic missions at the current president's request.
"I think that tradition ends with Trump," Mr Priess said. "It's based on courtesy and the idea that presidents may call on their predecessors for frank advice. I don't see Joe Biden calling up Trump to talk about intricate national security and intelligence issues. And I don't think Biden will send him anywhere as an emissary."
The last line of defence, like so many chapters in Mr Trump's presidency, would pose unprecedented considerations: criminal prosecution. The Espionage Act has been successfully used to convict current and former government officials who disclose information that damages US national security. It has never been used against a former president. But as of January 20, 2021, Trump becomes a private citizen, and the immunity he enjoys from criminal prosecution vanishes.
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