It’s not every day that you hear about the American president casually sharing an ally’s super-secret intelligence with Russian officials in the Oval Office. But was Donald Trump’s disclosure unprecedented? What exactly is new here, and what isn’t?

“It’s not unprecedented at all for presidents or national-security advisers or secretaries of state to share classified information with foreign officials,” especially those from allied countries, said Timothy Naftali, a presidential historian at New York University. The United States, for example, is part of an intelligence-sharing alliance with Australia, Canada, New Zealand, and the United Kingdom. As Joshua Keating notes in Slate, George W. Bush went so far as to invite several foreign leaders to sit in on customized versions of his daily intelligence briefing. These leaders included one Vladimir Vladimirovich Putin, during a high point in U.S.-Russian relations.

“It’s unusual to share sensitive intelligence from fragile sources with states [like Russia] that are not friendly,” Naftali added, but this too has been done before. As part of the Nixon administration’s opening to China, for example, Henry Kissinger wooed the Chinese by informing them that the United States was secretly providing military support to China’s ally, Pakistan, during the Indo-Pakistani War of 1971. The Reagan administration, for its part, shared intelligence with Saddam Hussein to avert an Iranian victory in the Iran-Iraq War.

Presidents, moreover, are unique among U.S. government officials in that they have the power to declassify material as they wish. In explaining to the American people why he’d decided to launch air strikes against Libya, Ronald Reagan publicized intelligence linking Muammar al-Qaddafi’s government to the bombing of a nightclub in West Berlin.

And then there’s perhaps the closest historical analogue to Donald Trump’s current predicament: Richard Nixon, in making the case during a press conference that North Korea had illegally downed a U.S. spy plane over international waters, revealed that the U.S. government was capable of intercepting Soviet and North Korean radar signals. “There was no uncertainty whatever as to where this plane was, because we know what [North Korea’s] radar showed,” Nixon told the media. “We, incidentally, know what the Russian radar showed. And all three radars [including America’s] showed exactly the same thing.”

The journalist Seymour Hersh documented what happened next:

The Nixon statement created near-pandemonium at the [National Security Agency]. “I died when I heard it,” one official said. …

After Nixon’s statement about the [spy plane], the NSA official says, “The Soviet Union and other countries changed every frequency, every crypt system, every net structure—all at once. It took months to work it out.” At the time of Nixon’s blunder, the Soviets, North Koreans, and Chinese were using relatively simple codes in their radar analyses and the NSA had been able to break those codes and recreate their radar patterns in its systems, giving the United States the incalculable advantage of knowing what the other side was seeing. …

[Down in the White House Situation Room, a staffer] attempted a weak joke: “We’re going to take the President’s clearances away.”

What makes Trump’s disclosures novel in recent American history is that, according to reporting by The Washington Post and others, they appear to have been made without much if any forethought, and drew on classified information gathered by another country rather than U.S. intelligence agencies. Naftali struggled to come up with precedents for such circumstances—at least any that are publicly known. The Post reports that Trump mentioned specific intelligence, involving an ISIS plot to exploit laptops on airplanes, to the Russian foreign minister and ambassador while gloating about the “great intel” he receives, and that White House officials then rushed to minimize the fallout from the remarks. The information, the paper notes, came from a U.S. partnerreportedly Israelthrough channels “so sensitive that details have been withheld from allies and tightly restricted even within the U.S. government,” and without the partner’s permission. (On Tuesday, Trump’s national-security adviser tried to defend the president by saying Trump “wasn’t even aware where this information came from.”)

Classified information is typically shared after considerable government deliberation since, as the Nixon anecdote illustrates, such disclosures risk undermining the work of intelligence officials and endangering national security. And it’s another matter entirely “when the United States shares another country’s secret information”as opposed to its ownwithout that country’s permission, Naftali explained. “The U.S. government does that very, very, very rarely” since it can jeopardize intelligence sources and the intelligence-sharing relationship as a whole.

“The key issue here is whether the president had a considered reason for sharing intelligence with the Russians and whether the source was so sensitive that the president put it at risk. And if so, did he put it at risk for a good reason?” Naftali asked. “Or was this the Oval Office equivalent of the late-night tweet—just trying to impress the Russians with something that he knew?” These questions are all the more urgent given that the intelligence was shared not with an ally but an adversary.

“Normally you share intelligence because you want something,” Naftali said. “So what did we want from the Russians? What did we get in return for telling them why we know that … terrorists have now figured out a way to make bombs out of laptops? Was there something that we wanted from them that required that we be precise about why we were sure that this was a new threat?”

It’s unclear “whether the president thought seriously about this and decided it’s going to benefit our future actions in Syria or in fighting ISIS if the Russians know why we are banning electronic devices from ... overseas flights,” Naftali told me. On Tuesday, Trump claimed that he shared the intelligence with the Russians for “[h]umanitarian reasons, plus I want Russia to greatly step up their fight against ISIS & terrorism.” But his administration still hasn’t clarified why he needed to take such extraordinary risks to accomplish these goals.