Of all the remarkable moments in the White House summary of the call between President Trump and the Ukrainian President, Volodymyr Zelensky, one stands out as particularly revealing. After telling Zelensky that “I would like you to do us a favor,” Trump said, “I would like you to find out what happened with this whole situation with Ukraine.” As the whistle-blower complaint explained, Trump was repeating a conspiracy theory that Ukraine, not Russia, had hacked the Democratic National Committee’s servers, in 2016, perhaps with the assistance of CrowdStrike, an American cybersecurity firm that investigated the D.N.C. breach. “I do not know why the President associates these servers with Ukraine,” the whistle-blower wrote. (No credible evidence of such a conspiracy has emerged—nor has an explanation for why Democrats would release private e-mails that damaged Hillary Clinton’s campaign and aided Trump.)

On Sunday, Tom Bossert, a former senior White House aide, told ABC’s “This Week” that he had informed Trump that the theory was false, and blamed Trump’s personal lawyer, Rudolph Giuliani, for feeding it to him. “I am deeply frustrated with what he and the legal team is doing, and repeating that debunked theory to the President,” he said. “It sticks in his mind when he hears it over and over again.” (Bossert later said, “In fairness, I don’t know that it was Rudy Giuliani that put that conspiracy theory into the President’s head,” but “I know somebody did.”)

Bossert’s frustration echoes the concerns of several current and former Trump aides. All American Presidents are the focus of myriad disinformation campaigns designed to influence their thinking. Domestic political rivals use public statements and private flattery to attempt to sway the President’s world view. Aides jostle to be seen as the most effective, resolute, and loyal lieutenant. But, under Trump, the amount of disinformation circulating in the White House has reached a new level, former Trump Administration officials told me. Trump is barraged with information from aides, political rivals, Fox News, and Twitter, much of it clearly designed to play to his long-held beliefs and biases. The former officials added that internecine battles among Trump’s White House advisers sometimes exacerbate the amount of faulty information being presented to the President.

Some aides also genuinely believe the conspiracy theories. This summer, a senior intelligence official told me that Trump’s Attorney General, William Barr, is convinced that there is something nefarious in how the F.B.I. started its investigation, in 2016, of possible ties between Trump campaign aides and Russian officials. This spring, Trump authorized Barr to carry out sweeping reviews of the investigation’s origins and of an Intelligence Community assessment that found that Russia intervened in 2016 to benefit Trump. In May, Barr appointed the U.S. Attorney for Connecticut, John Durham, to lead the review, but Trump and Barr have contributed to the investigation. This week, Justice Department officials confirmed that Barr had asked Trump to personally request that the leader of Australia aid Barr’s inquiry, and also that Barr and Durham travelled to Italy last week to ask intelligence officials there to assist the probe. Former Justice Department officials expressed surprise about Barr’s involvement. A senior intelligence official called the claims of an international plot—such as the Obama Administration asking British intelligence to surveil Trump Tower—“preposterous.” The official pointed out that the Republican-controlled Senate Intelligence Committee has already investigated the origins of the F.B.I.’s Trump-Russia investigation and found no wrongdoing.

Some former Trump Administration officials agreed that the President and some of his aides trafficked in conspiracy theories, but they also defended his policies. Although they recognized his personal flaws, they expressed support of his efforts to confront China, to force American allies to pay more for their own defense, and to prevent the U.S. from being drawn into another war in the Middle East. When presented with good information, they contended, the President makes good decisions. Others said that they had lost faith in the President and his policies after observing his decision-making. A former senior national-security official told me that Presidents George W. Bush and Barack Obama both did a better job of weighing information. “That’s why the President gets the big bucks,” he told me. “To discern where the preponderance of the evidence lies, to discern where the more persuasive argument lies.”

The former national-security official noted that disinformation had complicated Trump’s decision-making regarding policy in Afghanistan, in 2017. Some aides were telling Trump that he should adopt a proposal from Erik Prince, the brother of the Education Secretary, Betsy DeVos, to use private contractors rather than American soldiers to secure Afghanistan. The official said that the option was presented in an appealing but unrealistic way. “An American company will do this cheaper and better without young Americans in uniform dying,” he said, paraphrasing the argument made by backers of the plan. Trump was also told that China was making large amounts of money in Afghanistan, while the U.S. provided the country’s security, which played to Trump’s sense that America was being taken advantage of by the Chinese. In fact, both claims were highly misleading. Guards from Prince’s former private-security company, Blackwater, had been convicted of killing more than a dozen civilians in a 2007 shooting in Baghdad; deploying a large number of Blackwater-like security contractors likely would have fuelled a surge in anti-American sentiment among Afghans and aided the Taliban insurgency. The scale of Chinese profits was exaggerated. American defense contracts had, in fact, made far larger profits in Afghanistan than had Chinese firms. In the end, the proposal to use private contractors was rejected, and Trump kept thousands of U.S. troops in Afghanistan.

Last year, conspiracy theories from outside the White House nearly derailed the passage of a major piece of legislation. On January 10, 2018, the White House issued a statement supporting the renewal of Section 702 of the FISA Amendments Act, a post-9/11 program that allows U.S. intelligence agencies to collect the communications of foreigners overseas without a warrant—including when they are speaking with Americans. The next morning, on “Fox & Friends,” Andrew Napolitano, the network’s top judicial analyst, directly appealed to Trump to change his position on the bill. “Mr. President, this is not the way to go,” Napolitano said. “Spying is valid to find the foreign agents among us. But it’s got to be based on suspicion and not an area code.” Napolitano went on to invoke the debunked conspiracy theory that British intelligence agencies had spied on Trump Tower during the 2016 campaign. Napolitano had first floated the theory a year earlier; it was repeated by Sean Spicer, then the White House press secretary. Stunned British intelligence officials, in a rare public rebuke, dismissed the claim as “nonsense” and “utterly ridiculous.” The Republican-controlled Senate Intelligence Committee also investigated the claim and found it to be false.

Forty-five minutes after Napolitano asked Trump to oppose the law, the President tweeted, “This is the act that may have been used, with the help of the discredited and phony Dossier, to so badly surveil and abuse the Trump Campaign by the previous administration and others?” Fearing that the legislation would fail to pass without Trump’s support, aides convened an Oval Office meeting. John Kelly, then the White House chief of staff; H. R. McMaster, then the national-security adviser; Dan Coats, then the director of National Intelligence; Mike Pompeo, then the C.I.A. director; and Paul Ryan, then the Speaker of the House, who joined by phone, convinced Trump to reverse his reversal. “Today’s vote is about foreign surveillance of foreign bad guys on foreign land,” he tweeted. “We need it! Get smart!”

Former officials said that they came to the conclusion that the character flaws they saw while working for Trump prevent the President from delivering on his agenda. They said that the President makes false statements, distrusts his own staff at times, and looks to outsiders for guidance. Now the pursuit of a conspiracy theory could lead to Trump’s impeachment. Under attack, Trump is defending himself in the way he knows how—by claiming that the whistle-blower complaint is itself a conspiracy. On Wednesday, he said that he believed Representative Adam Schiff, the Democratic chair of the House Intelligence Committee, “helped write” the complaint, and dismissed it as a “scam.” Schiff flatly denied Trump’s claims.