When a delegation of senior Israeli officials visited the Trump White House on February 13, 2017, they wanted to discuss several issues with their new American counterparts. Topping the list was a secret letter concerning a subject the Israelis had promised the Americans never to discuss publicly—Israel’s undeclared nuclear arsenal. In a recent piece for The New Yorker, I described a tense scene in the West Wing as the Israeli delegation—which included Israel’s Ambassador to the United States, Ron Dermer—tried to get the letter signed by President Donald Trump. By all accounts, the American Administration was eager to please the Israeli Prime Minister, Benjamin Netanyahu, whom Trump had promised to lavish with unprecedented support. But, at that chaotic moment, Trump’s aides felt blindsided by the Israeli request. They knew nothing about the existence of any letters and were confused by the sense of urgency coming from the Israelis. The Americans had other pressing concerns—later that day, Michael Flynn, the national-security adviser, would hand in his resignation letter—and they didn’t appreciate feeling as though the Israelis were telling them what to do. “This is our fuckin’ house,” one of the Americans snapped.

The White House’s reaction was understandable. There had been a similar moment of surprise eight years earlier, when Barack Obama became President and received a similar request. The very existence of the letters had been a closely held secret. Only a select group of senior American officials, in three previous Administrations, knew of the letters and how Israeli leaders interpreted them as effectively an American pledge not to press the Jewish state to give up its nuclear weapons so long as it continued to face existential threats in the region. (American officials say the letters weren’t that explicit and fell short of constituting a binding commitment.) When Trump’s aides moved into the White House, they didn’t find any copies of the previous letters left behind by their predecessors. The documents had been sent to the archives. The Israelis, however, had copies.

Israel crossed the nuclear threshold on the eve of the Six-Day War, in 1967. At that time, it had three nuclear devices, according to Avner Cohen, a nuclear historian at the Middlebury Institute of International Studies at Monterey and the author of two books on the origins of Israel’s nuclear program. Israeli efforts to build a bomb at the nuclear complex in Dimona had been a source of tension with Washington for nearly a decade. But, by the fall of 1969, when Golda Meir, Israel’s Prime Minister, met with Richard Nixon at the White House, Israel’s possession of nuclear weapons was a fait accompli and the two sides reached an unwritten understanding: the Israelis would not declare, test, or threaten to use their nuclear weapons, and the Americans would not pressure the Israelis to sign a landmark international nuclear-nonproliferation treaty known as the N.P.T. (Israel never became a signatory, and U.S. efforts to inspect Dimona stopped.)

Successive Israeli governments abided by the arrangement, which, in Hebrew, is referred to as “amimut,” which means opacity. In English, the arrangement is often referred to as Israel’s “policy of ambiguity.” A joint document describing the agreement was never prepared. Instead, each side relied on its own notes, a former official said. President Gerald Ford abided by Nixon’s deal. Israeli officials were concerned that Jimmy Carter would chart a different course, but the American position, through the Carter and Reagan Administrations, remained unchanged.

The Israelis first started to feel as though the unwritten Meir-Nixon arrangement was no longer sufficient during the Presidency of George H. W. Bush, when, after the first Gulf War, in 1991, world powers talked about the possibility of creating a zone in the Middle East free of weapons of mass destruction, including nuclear arms.

The first iteration of the secret letter was drafted during the Clinton Administration, as part of an agreement for Israel’s participation in the 1998 Wye River negotiations with the Palestinians. In the letter, according to former officials, President Bill Clinton assured the Jewish state that no future American arms-control initiative would “detract” from Israel’s “deterrent” capabilities, an oblique but clear reference to its nuclear arsenal. Later, Israeli officials inserted language to make clear to Washington that Israel would “defend itself, by itself,” and that it would, therefore, not consider the American nuclear arsenal to be a substitute for Israeli nuclear arms. George W. Bush, when he became President, followed Clinton’s lead, signing a similar letter, former officials told me.

Then, in 2009, a new President, Barack Obama, took office. Almost from the start, Netanyahu was distrustful of Obama, and vice versa. “With Obama, we were all crazy,” an Israeli official told me. That April, Obama delivered an aspirational speech in Prague, setting out “America’s commitment to seek the peace and security of a world without nuclear weapons.” Obama’s advisers subsequently learned “how paranoid Bibi was that Obama was going to try to take away Israel’s nuclear weapons,” a former U.S. official told me, adding, “Of course, that was never our intent.” Obama signed an updated version of the letter in May, 2009.

While Israeli officials interpreted the letters as an effective commitment by successive American Presidents not to pressure Israel regarding its nuclear arsenal, U.S. officials told me that they viewed the letters as less categorical. “It was not a blanket ‘We’ll never ask Israel to give up its nuclear weapons.’ It was more ‘We accepted the Israeli argument that they’re not going to disarm under current conditions in the Middle East,” a former U.S. official told me. Avner Cohen, the Middlebury Institute historian, said that U.S. Administrations have been reluctant to give up entirely on the possibility of ridding the region of nuclear weapons if Israel were to reach a comprehensive peace agreement with its neighbors, including Iran.

Ahead of a nonproliferation conference in 2010, Netanyahu became concerned, once again, that Israel could come under international pressure to disarm. In response, Obama made a public statement that echoed the contents of the secret letters, without revealing their existence. “We discussed issues that arose out of the nuclear-nonproliferation conference,” Obama said, after meeting with Netanyahu on July 6, 2010. “And I reiterated to the Prime Minister that there is no change in U.S. policy when it comes to these issues. We strongly believe that, given its size, its history, the region that it’s in, and the threats that are levelled against . . . it, that Israel has unique security requirements. It’s got to be able to respond to threats or any combination of threats in the region. And that’s why we remain unwavering in our commitment to Israel’s security. And the United States will never ask Israel to take any steps that would undermine their security interests.”

The tense scene in the West Wing over the letter came on the heels of a particularly chaotic transition, from Obama to Trump. Their advisers distrusted one another, and it is unclear if they ever discussed the Israeli letters before the Inauguration. So when Ambassador Dermer came to the White House to talk to Michael Flynn about arranging for Trump to sign the letter, Trump’s aides were confused and, initially, said that they needed more time. U.S. officials said that the Israelis wanted to limit who could take part in discussions of the letter, citing the need for secrecy. The Americans pushed back. Afterward, senior White House officials huddled together and complained to one another that Dermer had acted as though he owned the White House. Dermer declined to comment on the letter and told me that he does not recall any cursing. Flynn was ousted that night. Later, Trump signed the letter, becoming the fourth U.S. President to do so.

Like Obama’s advisers, Trump’s aides were baffled by the importance that Netanyahu placed on getting the letters signed so quickly. Cohen said that the issue is central for Netanyahu because the nuclear arsenal fuels his “sense of impunity, sense of Israel being so powerful, that it can dictate its own terms in the region and beyond.”