Frederick Forsyth once wrote that “The spies in history who can say from their graves, the information I supplied to my masters, for better or worse, altered the history of our planet, can be counted on the fingers of one hand.” Although his story had been mostly unknown so far, Ashraf Marwan, the son in-law of Egyptian President Gamal Abdel Nasser, a close advisor of his successor President Anwar Sadat, and a Mossad spy for 28 years, is one of these few.

Born to a well-respected family in Cairo in 1944, Marwan made his way to the top by marrying Nasser’s daughter, Mona, in 1966. In 1969 he started working at the Presidential Palace, first under his father-in-law and, after Nasser’s death in September 1970, as Sadat’s close aide. In late 1970, motivated by a combination of ego, greed, and a need for risk-taking, he offered his services to the Israeli Mossad. Within a short period of time, under the codename “The Angel,” he became Israel’s greatest spy, and one of the most important in modern history.

Two main factors contributed to Marwan’s importance. One was the strategic context of Egyptian-Israeli relations: In the wake of the War of Attrition along the Suez Canal, which ended in 1970, Egypt began preparing for war with the aim of retaking the Sinai Peninsula that it had lost to the Israelis in the 1967 Six Day War. Second, Marwan had unparalleled access to his nation’s best-kept secrets. Serving as Sadat’s aide-de-camp, he was ideally positioned to provide Israel with information about the coming war—including the full Egyptian war plans, detailed accounts of miiltary exercises, original documentation of Egypt’s arm deals with the Soviet Union and other countries, the Egyptian military Order of Battle, the minutes from meetings of the high command, accounts of Sadat’s private conversations with other Arab leaders, and even the protocols of secret summit meetings in Moscow between Sadat and Soviet Premier Leonid Brezhnev.

All this information made its way to the desks of Israeli Prime Minister Golda Meir, Defense Minister Moshe Dayan and the Israeli Defense Force (IDF) chief of staff in raw form. By providing these documents as well as his own assessments, Marwan turned his country, Israel’s main enemy, into an open book, shaping Israel’s strategic and tactical approach to Egypt and allowing the Israelis a direct look at Egypt’s war calculus, including Sadat’s minimal requirements for launching a war. To go to war, they learned, Sadat believed he needed to get his hands on long-range attack aircraft and Scud missiles, without which he would not be able to overcome Israeli air superiority.

After years of efforts to acquire them, however, Sadat made the decision in October 1972 to go to war anyway, together with Syria, without these weapons. Marwan reported on this decision, and the year of intensive military preparations that followed, as well. Yet for some reason, key figures in Israel’s intelligence community continued to believe that Sadat would not go to war without possessing the weapons needed to neutralize Israeli air superiority. Their mistake led Israel to the brink of catastrophe when war was finally launched on October 6, 1973.

The Yom Kippur War, the most intense confrontation in the history of the Arab-Israeli conflict, is also the most traumatic event in Israeli history. In the first twenty-four hours of battle Israel’s defense line along the Suez Canal fell, allowing Egypt to massively penetrate the Sinai; on the Syrian front, about half the territory of the Golan Heights was lost as well. More than 500 Israeli soldiers lost their lives during the first day, and the total number of soldiers killed would approach 3,000—as a proportion of the small population, it would have been the equivalent of losing 180,000 American soldiers in combat. On the eve of battle, Israel had seen itself as an unrivalled regional power; less than twenty-four hours later, Defense Minister Dayan spoke about “the fight for the ‘Third Temple.’ ” Just as Jerusalem’s First Temple had been destroyed in 586 BCE and the Second Temple in 70 CE, the third Jewish commonwealth was again under threat of destruction.

What prevented an Israeli defeat was a last-minute warning Marwan delivered to Zvi Zamir, chief of the Mossad, in an emergency meeting in London on October 5: “War will break out tomorrow.” Here Marwan proved to be more valuable than all of Israel’s most sophisticated surveillance equipment, more precious than the entire network of spies the Israelis had spent years cultivating, and in some respects more useful even than Israel’s own intelligence chiefs. Because of this warning, Israel began mobilizing its forces on the morning of October 6—hours before the attack was launched. Without his warning, we now know, the entire Golan Heights would have been lost, and the war would have ended with far greater Israeli losses of both territory and life.

Ultimately, the 1973 war ended in a kind of a draw. Thanks to Marwan’s intelligence, the IDF was able to recover from its initial defeats and reached a military victory on both fronts, though neither the Egyptian nor the Syrian army collapsed. The cost of the war led Israel to return the whole of Sinai to Egypt in exchange for peace—a deal that prior to the war had been explicitly rejected by the Israelis. Egypt, strengthened by what it saw as victory, left the Soviet camp and became an American ally, marking an important Soviet retreat in the Middle East. Tensions rose between Washington and Moscow, putting a pause on détente, and the Arab oil embargo that was launched during the war, purportedly in response to American assistance to Israel, led to a global economic recession during the 1970s.

Both the positive and negative consequences of the war were the result of the fact that it ended in a draw. Marwan played a central role in that outcome. And in this sense, he may be counted among the tiny number of spies who can say from their graves: “the information I supplied to my masters, for better or worse, altered the history of our planet.”

The Angel is his story.