[Tom Segev, a columnist for the Israeli newspaper Haaretz, is the author, most recently, of “1967: Israel, the War and the Year That Transformed the Middle East.”]

FORTY years ago today, on the morning of June 5, 1967, Jordan launched an artillery attack on the Israeli part of Jerusalem. In reaction Israel conquered the Arab sections of the city as well as the West Bank.

History is full of “what ifs,” and responsible historians should not indulge in such speculation. But journalists may. What if Israel hadn’t taken East Jerusalem and the West Bank in the Six-Day War? Would the Palestinian situation have found some solution and Israel be living at least in relative peace with its neighbors? Would Islamic fundamentalism and terrorism have been avoided?

Perhaps. But the alternate history is not as outrageous or inconceivable as one might think. Leading Israeli policy planners had determined six months before the Six-Day War that capturing the West Bank would be bad for the country. Recently declassified Israeli government documents show that according to these policy planners, taking over the West Bank would weaken the relative strength of Israel’s Jewish majority, encourage Palestinian nationalism and ultimately lead to violent resistance.

These comprehensive political and strategic discussions began in November 1966 and concluded in January 1967. The participants were representatives of the Mossad, the Israel Defense Forces’ intelligence branch and the Foreign Ministry. The documents they prepared were approved by Prime Minister Levi Eshkol and the army’s chief of staff, Yitzhak Rabin, and therefore reflect Israel’s strategic thinking six months before the war.

There was general agreement that it would be to Israel’s advantage for King Hussein of Jordan, whose country controlled the West Bank, to remain in power: he had, in effect, accepted Israel’s existence, so Israel naturally had an interest in strengthening his regime....