Israel has managed to secure a written pledge from four successive U.S. presidents to safeguard its presumed nuclear deterrent, The New Yorker magazine reported on Monday.

According to uncorroborated reports in the foreign media, Israel has as many as 200 nuclear warheads as part of a presumed military nuclear program dating back to the 1960s. Israel has never publicly acknowledged these reports.

Israel has also pledged not to be the first nation to introduce nuclear weapons in the region.

According to Monday’s report, in the wake of the first Gulf War in the early 1990s, Israel felt that the unwritten understanding struck between U.S. President Richard Nixon and Israeli Prime Minister Golda Meir in the early 1970s to ensure Israel would never be compelled to denuclearize was insufficient.

Eventually, Israeli policymakers convinced U.S. President Bill Clinton to put the Nixon-Meir understandings into writing.

“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,” said the report, by The New Yorker’s Adam Entous.

“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 [alleged] nuclear arsenal.”

The letter was later signed by President George W. Bush. But when President  Barack Obama won office in 2008, Israel was concerned that he would hold off on renewing the pledge.

“With Obama, we were all crazy,” an Israeli official was quoted in the report. A former U.S. official is quoted as saying that Obama’s advisers believed Israeli Prime Minister Benjamin Netanyahu was “paranoid” that the United States would try to take away Israel’s presumed nuclear weapons, but that “wasn’t our intent.”

Ultimately, Obama signed “an updated version of the letter.”

According to the report, efforts to renew the pledge when Trump assumed office initially stalled, when Israeli Ambassador to the United States Ron Dermer made the request in a surprise move in February 2017. When he came to the White House, the Trump officials said they needed more time, but the Israelis “wanted to limit who could take part in discussions of the letter, citing the need for secrecy.”

According to the report, part of the tensions then arose because the White House was not aware of the letters.

“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,” the report said.

When Trump became president, his aides “didn’t find any copies of the previous letters left behind by their predecessors. The documents had been sent to the archives.”