The Iranian president, Mahmoud Ahmadinejad, has done Israel a favor by denying the Holocaust and demanding that Israel be wiped off the map or moved to Alaska. With his recent pronouncements, Mr. Ahmadinejad has reminded us not only why a nuclear Iran cannot be tolerated, but also exactly why a nuclear Israel has been and will continue to be tolerated. If the justification for other nations’ deterrents today is power and prestige, the justification for Israel’s deterrent is to deter. Its survival depends upon the nuclear option. Alone among nations, Israel faces a permanent threat to its very existence.

Hence Michael Karpin’s history of Israel’s nuclear program (Simon & Schuster, 416 pages, $26) comes at an opportune moment. Mr. Karpin is a highly experienced Israeli journalist, and his book demonstrates that he knows the political and security establishment of his country inside out. Unlike the technician Mordechai Vanunu, whose zeal to expose the nuclear secrets with which he had been entrusted at the Dimona plant persists even after his release from 18 years of imprisonment for treason, Mr. Karpin has no ideological agenda and is content to work within the restrictions imposed by Israeli law.

These restrictions do not prevent him telling the story in as much detail as it requires, though he evidently knows more about Israel’s nuclear capability than he can divulge. In response to Pakistan’s “Islamic bomb” and the threat of nuclear proliferation to nearer and even more hostile neighbors, in particular Iran, Israel has added a submarine-borne deterrent to its arsenal of nuclear-armed aircraft and land-based missiles.

The official policy remains one of nuclear ambiguity, a doctrine developed by the man who emerges from this book as the father of the Israeli bomb: Shalhevet Freier. It was Freier who in 1956 was sent to Paris as an undercover “scientific attache,” with the aim of persuading the French to help Israel to build a nuclear bomb. In the aftermath of the Suez crisis, with Israeli forces still occupying the Sinai desert and with the Eisenhower administration as well as the Soviet Union putting pressure on Prime Minister Ben-Gurion to withdraw, the French saw an opportunity to bolster their crumbling position in North Africa by helping Israel to build a nuclear reactor at Dimona in the Negev desert.

The United States and Britain, the only other Western nuclear powers, were reluctant to let Israel build more than a small research reactor, with no facility to produce the plutonium necessary to make weapons. But the French, worried about a million French Algerian colonists surrounded by Arabs, saw the Egyptian dictator Nasser as a common threat to France and Israel.

Shimon Peres, who was one of the Israelis involved in the negotiations, told Mr. Karpin that the French sympathized with fellow victims of the Nazis: “They identified with us.” This sympathy was undoubtedly a factor for a few (mainly Jewish) officials, but a more important motive was surely access to Israeli intelligence, by then already second to none in the Middle East.

The deal was settled in the last days of the Fourth Republic, when French governments lasted weeks or days, but was sustained by dedicated bureaucrats and scientists for years, even after General de Gaulle came to power in 1958. Officials at first deceived de Gaulle, flouting presidential orders to end cooperation with Israel.

In 1960 he paid a visit to the French plant at Saclay and spotted a tiny Citroen Deux Chevaux car with red diplomatic license plates. It belonged to an Israeli scientist at work on the project. De Gaulle demanded to know what a foreigner was doing in the most secret laboratory in France, just two months after the first French nuclear device had been tested in the Sahara. When he was told, there was almost a nuclear explosion in the president’s Elysee Palace, and an embargo was placed on nuclear assistance. In practice, contracts were honored, and Israel was able to complete the Dimona plant, designed to produce enough plutonium to make 10 bombs a year.

France was not the only country that helped Israel: Norway supplied heavy water. But from 1960, when the existence of the Dimona facility was disclosed by the New York Times, leaked by the CIA, what mattered was the attitude of the United States. The Israelis need not have worried. The Eisenhower administration decided not to obstruct the Israelis, or even to stop American Jews from financing the project. Kennedy even told Ben-Gurion that he was elected by Jewish votes.

The pretense that Dimona was not intended for military purposes suited both sides. It was vital for Israel that the United States should not link conventional weapons sales to nuclear issues. And it was vital for the United States that Israel should not provoke the Soviet Union into arming the Arabs with nuclear weapons. The United States made sure Israel could defend itself without using its deterrent, and Israel’s bomb in the basement did its work merely by existing.

The Six-Day War and the Yom Kippur War might have precipitated a superpower confrontation like the Cuban Missile Crisis, but even on the brink of defeat in 1973, Prime Minister Golda Meir never threatened to use nuclear bombs. Even so, Egypt’s President Sadat secretly assured Henry Kissinger, then secretary of state, that his aim was not to annihilate Israel. There was an angry exchange between Moscow and Washington, but Mr. Kissinger had no need for brinkmanship.

Mr. Karpin concludes with a chilling scenario: war with Iran. An Israeli attack on the two main Iranian nuclear facilities at Natanz and Arak has been facilitated by the supply of “bunker buster” bombs from the United States. Iran could, Mr. Karpin thinks, respond only by activating Hezbollah and Palestinian Arab terrorists, who could do considerable damage to Israeli civilians but present no existential threat.

But there is, he argues, an alternative: a comprehensive Middle East peace treaty, including Iran, Iraq, and Libya, followed by a long process of confidence-building and cooperation. Only then would negotiations on arms control and disarmament begin. Last of all, nuclear weapons could be banned from the entire region. He points out that the fathers of the Israeli bomb always intended it to be a means to force the Arabs to accept a final peace settlement with Israel. Just as the Allies originally developed nuclear weapons to pre-empt the Axis, so Israel justifies its regional nuclear monopoly by the fact that it alone faces the threat of liquidation.

My own view is that it is utopian to suppose Israel will ever willingly give up its deterrent. How could Israel be sure that an “Islamic bomb” would never be used against it? The danger that a state may suddenly become hostile is exemplified by the attitude of France. Can anyone imagine Jacques Chirac helping Israel to develop nuclear weapons today? It is more likely that the French would arm Israel’s enemies.

Israel cannot be blamed for refusing to rely for its current safety on the very countries, including even the United States, that failed to save European Jewry from the Nazis.To have survived one genocide is more than enough reason to want to deter anybody from trying again.

Mr. Johnson’s “London Letter” appears Thursdays in The New York Sun.