Since former FBI deputy director Andrew McCabe began promoting his new book “The Threat,” there’s been an understandable focus on something that isn’t even mentioned in it: Deputy Attorney General Rod J. Rosenstein’s reported comment in May 2017 that perhaps it was time to consider exploring the 25th Amendment’s provisions for removing a president from office.

In interviews, McCabe has indicated that the conversation went no further than Rosenstein’s comment and, despite President Trump’s base-boosting tweets about treason and coups, the comments from Rosenstein were no more a coup than hypothetical speculation from another senior administration official about Trump’s impeachment might be.

In the abstract, in this moment some 18 months removed from the reported comments, the idea of senior officials reaching that level of concern about Trump can seem melodramatic or nefarious. But the comments weren’t made now, more than a year after special counsel Robert S. Mueller III got to work and well after the various revelations about Trump and his campaign have been absorbed into our collective consciousness.

It’s worth instead putting ourselves into the more pertinent context of that moment, remembering what was and wasn’t publicly known about the investigation and how the investigation itself had evolved to the point at which Rosenstein allegedly made his comments.

On July 31 of the previous year, the FBI had opened a counterintelligence investigation that stemmed from Trump campaign adviser George Papdopoulos’s revelation to a foreign diplomat that he was aware in May that Russia had a cache of emails incriminating Trump’s 2016 opponent Hillary Clinton. About a week before that investigation was opened, a number of documents stolen from the Democratic National Committee were released by WikiLeaks with the apparent aim of disrupting the Democratic convention that was about to begin.

In March 2017, then-FBI Director James B. Comey described the investigations in testimony before Congress — the first time they were acknowledged in public.

“I have been authorized by the Department of Justice to confirm that the FBI, as part of our counterintelligence mission, is investigating the Russian government’s efforts to interfere in the 2016 presidential election,” he said, “and that includes investigating the nature of any links between individuals associated with the Trump campaign and the Russian government and whether there was any coordination between the campaign and Russia’s efforts.”

Eventually, we would learn that four individuals were being investigated in particular: Papadopoulos, who had ongoing communications with someone linked to the Russian Foreign Ministry; another campaign adviser named Carter Page, who traveled to Moscow in July 2016 and had a conversation of some kind with a Russian deputy prime minister; campaign adviser Michael Flynn, who had contact with Sergey Kislyak, Russian’s then-ambassador, and who had dined with Russian President Vladimir Putin at an event in December 2014; and Paul Manafort, the campaign’s onetime chairman, who had extensive ties to Russia through his work in Ukrainian politics.

Where each of these investigations was and how much was known when Trump was inaugurated are not clear. In his book, though, McCabe notes that a “confidential informant with preexisting tangential ties to people associated with Trump’s political operation” had provided information about “specific national-security risks” involving Russia’s involvement in the campaign. This is apparently a reference to Stefan A. Halper, who worked with the FBI’s investigation into the Trump campaign by meeting with Papadopoulos and Page. He had previously worked with Flynn at a seminar in early 2014.

The FBI had other information about national security risks, too: The dossier of reports from former British intelligence officer Christopher Steele, compiled as part of his work for a firm hired by a law firm that represented Clinton’s campaign and the Democratic Party. Steele had worked with the FBI in the past and provided information from his reports to the Bureau.

Again, some part of this information, if not much of it, was in the agency’s hands prior to the election. The Bureau and the government more broadly were already aware that Russia was working directly to try to undermine the election, by exploring vulnerabilities in state elections systems (prompting an unusual warning in October 2016) and by trying to influence American voters over social media. What’s more, it was already believed that Russia was behind the hacking of the DNC.

And, of course, there was Trump himself. Over the course of the campaign, he repeatedly had offered praise for Putin and had deflected allegations about the hacked documents that were being steadily released by insisting that the perpetrator could have been Russia, China — or a heavyset guy working from his basement in New Jersey. At one point in July, he infamously called on Russia to release emails it might have stolen from Clinton’s private email account. (That same day, Russian hackers tried to breach Clinton’s server, according to an indictment obtained by Mueller last year.)

Once Trump won the election, concern about his relationship with Russia prompted an unusual effort. The New York Times reported in March 2017 that officials in the administration of President Barack Obama (though not acting at his direction) undertook a concerted campaign to share information about Russia’s interference efforts throughout the government in order to ensure that it was not destroyed or lost under the incoming administration.

“There was also an effort to pass reports and other sensitive materials to Congress,” the Times reported. “In one instance, the State Department sent a cache of documents marked ‘secret’ to Senator Benjamin Cardin of Maryland days before the Jan. 20 inauguration.”

The Obama administration had already prepared a report detailing what it knew about Russian interference. It included assessments that the effort was focused, in part, on ensuring Trump’s election. That information was transmitted to Trump during a Jan. 6, 2017, meeting at Trump Tower.

“The evidence included texts and emails from Russian military officers and information gleaned from a top-secret source close to Mr. Putin, who had described to the C.I.A. how the Kremlin decided to execute its campaign of hacking and disinformation,” the Times later reported.

Trump nonetheless publicly dismissed the link between the hacked material and Russia. In a news conference days later, he again hinted that the hacking had perhaps been done by China.

That January briefing was the first time that Comey met Trump. It was also the first time that Comey memorialized one of his interactions with Trump. Comey also wrote memos after conversations with Trump in late January and early February. In both of those memos, Comey indicated that Trump referred to conversations with Putin. In the first, Comey documented that Trump asked him for his loyalty.

The most important of those memos, though, documented a conversation that Comey had with Trump on Feb. 14, 2017. That was the meeting during which Comey says Trump asked him to drop the counterintelligence investigation into Flynn, who had recently resigned as national security adviser following revelations that he’d had conversations with Kislyak about which he had allegedly lied to Vice President Pence. (Obama, in a meeting with Trump in November 2016, was concerned enough about Flynn to recommend that Trump not give him an administration position.)

In another memo, from the end of March, Comey documented Trump asking him how the “cloud” of the Russia investigation could be lifted from his presidency. When Trump made the same point in April, Comey says he referred Trump to the Department of Justice. That leadership was in flux: Then-Attorney General Jeff Sessions had recused himself from the Russia probe (to Trump’s frustration) on grounds of his role in the Trump campaign, and Trump’s nominee for the deputy position, Rosenstein, was not confirmed until the end of April.

All of the above is information that was available to the Justice Department at the beginning of May 2017. The question of Trump and Russia, though, soon became much more urgent.

On May 3, 2017, Comey testified before Congress again — testimony that apparently accelerated Trump’s frustration with his tenure.

On May 8, Rosenstein — only a few weeks into the job — was called to the White House, where Trump presented him with a letter he had written that would fire Comey. Trump asked Rosenstein to write a memo making the case for firing Comey, according to McCabe; he did so.

It did not include one part that Trump sought to include, according to McCabe: A comment from Rosenstein that Comey was being fired in part because of the Russia investigation. A line to that end — that Comey had told Trump three times that the president was not under investigation — instead made it into a note that Trump wrote to Comey. It was transmitted to the FBI along with a letter from Sessions that actually served as Comey’s termination.

On May 9, the letter was delivered to the Justice Department. Comey was out, catching him (and McCabe) by surprise. McCabe encapsulates his response in his book.

“The firing of Jim Comey gave new urgency to the FBI’s investigation of Russian interference in the 2016 elections — that interference was a fact, not a supposition — and into possible collusion by the Russians with the Trump campaign,” McCabe writes.

That evening, McCabe was called to the White House. By now, he had seen the memo from Rosenstein and the note from Trump. (He opened it because Comey never received it; he was in California giving a speech.)

During that conversation, Trump asked McCabe if he was aware that Comey had told him he was not under investigation.

“At the forefront of my mind was the president’s termination letter to Director Comey, which I had just read,” he writes. “I kept thinking about that oddly prominent mention of this concern the president was now raising, again, about whether he was being investigated.”

That would be further reinforced. On May 11, Trump was interviewed by NBC News. He told the network’s Lester Holt that while considering firing Comey, the Russia investigation was part of his thought process.

On the day between that interview and Comey’s firing, something else of significance happened. Trump invited Russian Foreign Minister Sergei Lavrov and Ambassador Kislyak to an unusual meeting in the Oval Office, a meeting that, it would later be revealed, stemmed from a request by Putin.

During that meeting, Trump reportedly told the Russians that he had fired Comey (who was “crazy, a real nut job”) and that while he had “faced great pressure because of Russia,” the firing meant that “that’s taken off.” This was reported publicly on May 19 but was likely known inside the administration earlier. On May 15, The Post reported that Trump had disclosed classified information during his meeting with Lavrov and Kislyak.

That was an important day, as it turned out. On May 15, 2017, McCabe met with the team investigating Russia’s interference effort, and they discussed opening investigations into the president for collusion and obstruction of justice. This may have been the point at which the FBI decided to enact a plan to protect the Russia investigation, as reported by the AP on Tuesday.

“The president’s possible connection to obstruction was no longer limited to his having been the leader of a campaign, some of whose members may have crossed a line in various ways,” McCabe writes. “Now the president himself had fired the director.”

This was also apparently the day on which Rosenstein allegedly told McCabe that the 25th Amendment might be needed to remove Trump from office. (It’s not identified as such in McCabe’s book, but he indicates that it was at a meeting on May 15 that Rosenstein “wondered aloud if there was some way to collect explicit evidence of the president’s apparent motivations” — an apparent reference to Rosenstein’s allegedly proposing wearing a wire to meet with Trump.)

Comey himself reentered the mix with a Times report on May 16 that detailed his conversation with Trump about the firing of Flynn. The next day, Rosenstein signed the order appointing Mueller as special counsel.

As importantly, though, it was also the day on which Rosenstein and McCabe traveled to Capitol Hill to brief congressional leaders on the boundaries of the Russia investigation. No one who attended expressed concerns about the investigation or Mueller, McCabe writes — including Rep. Devin Nunes (R-Calif.), whose allegiance to Trump was already established.

McCabe writes that he thought the briefing was particularly important in case others in the Justice Department were fired in an effort to end the probe.

“The Russia investigation was on solid ground,” McCabe writes about that briefing. “Everybody who needed to know about it knew about it. If the investigation ever got wiped away, that would involve forces beyond my control. It could not be struck from the record. All the steps we took were fully documented. If anyone tried to close it down, it could not be done in secret.”

Over the months that followed, an inordinate amount of additional information emerged. About Manafort’s interactions with a Russian believed to be linked to Russian intelligence. About a meeting between top Trump campaign staffers and a Russian lawyer at Trump Tower. About murky connections between Trump adviser Roger Stone and WikiLeaks. About the hacking and social media efforts by the Russians, documented in detail by Mueller’s team. About who knows what else that might emerge when Mueller’s work is done.

At the time of Comey’s firing, though, the feeling within the Justice Department and FBI seems to have been clear: What they already knew from July 2016 forward was enough to make them very, very worried.