Forty years ago this week, Israel won the Six Day War and reunified the city of Jerusalem. Not only had the map of the Middle East been changed, but a completely new international legal reality emerged that unfortunately has been largely forgotten.

After most armed conflicts, the international community has sought to re-establish the status quo ante — the previous situation — as part of a political settlement. However, many aspects of the prewar status quo in 1967 were untenable, if not illegal. Jordan and Egypt previously occupied the West Bank and Gaza Strip as a result of an invasion by the Arab states in 1948 that the U.N. Secretary-General at the time, Trygve Lie, called an act of “aggression.”

In the Six Day War nearly 20 years later, Israel entered these territories in what was plainly a war of self-defense. Indeed, before Israeli forces moved in Jerusalem, Jordanian artillery fired nearly 6,000 artillery shells on the Israeli parts of the city. When many of the leading authorities on international law at the time examined Israel’s rights in Jerusalem in relation to its previous occupants, they reached a clear conclusion supporting Israel’s postwar claims. For example, Stephen Schwebel, who later became the president of the International Court of Justice in The Hague, would write in 1970: “When the prior holder of territory had seized that territory unlawfully, the state which subsequently takes that territory in the lawful exercise of self-defense has, against that prior holder, better title.”

Israel legal rights in 1967 were again reflected a half year later in the language of the U.N. Security Council Resolution 242 that would become the cornerstone of every Arab-Israel peace agreements for decades thereafter. As is well known, the resolution, which was drafted by the British and American delegations at the U.N., did not call on Israel to withdraw from all the territories it captured in the Six Day War, but rather “from territories.” Britain’s foreign secretary in 1967, George Brown, clarified the meaning of the phraseology that was ultimately chosen: ” … Israel will not withdraw from all the territories.”

According to declassified U.S. cable traffic, the pressure to put the word “all” in the withdrawal clause came from the Kremlin directly. To buttress their demand the Soviets sought to have Israel branded as the aggressor in both the Security Council and in the General Assembly, but they utterly failed. President Johnson held firm against all these efforts to compromise Israel’s security, recognizing, as he would later admit, that a return to the pre-war situation was a recipe for further conflict. For that reason, Resolution 242 called for establishing “secure” boundaries, with the understanding that the pre-war lines were neither secure nor recognized.

Resolution 242 did not call for a restoration of the status quo ante in Jerusalem, where the entire Jewish population of the Old City had been forcibly evicted in 1948 and over 50 of its synagogues had been destroyed or desecrated. Going back to the pre-war situation, during which Jews were prevented from visiting the Western Wall to pray, would mean reinstating the loss of religious freedom in Jerusalem. The U.S. ambassador to the U.N. in 1967, Arthur Goldberg, disclosed, in fact, that “Resolution 242 in no way refers to Jerusalem and this omission was deliberate.” In 1988, the secretary of state, George Shultz, plainly declared the policy of the Reagan administration that Israel “will never negotiate from or return” to the 1967 lines. And in April 2004, President Bush wrote to Prime Minister Sharon in that spirit that Israel had a right to “defensible borders” instead of the old armistice lines that the Arab states had violated.

Today, the Palestinian Authority and Hamas have largely replaced the Arab states as claimants to the territories that Israel captured and have been disputed since 1967. It was expected that with these changes, Israel’s rights might have received a more fair hearing. After all, who would suggest placing the holy sites of Jerusalem under the Hamas, whose ideological cousins are attacking churches and mosques across the Middle East? Moreover, it is transparent today that if Israel were to withdraw from its long-held security positions along the Jordan Valley barrier, Al Qaeda affiliates that are today penetrating Lebanon, the Gaza Strip, and even Hashemite Jordan, would seize the opportunity presented and unleash a wave of jihadi volunteers and new arms, who would certainly escalate the present attacks against Israel.

Yet, perhaps out of desperation for something positive in the Middle East, Western diplomats are now embracing the “Saudi Peace Initiative,” which, if implemented, would strip Israel of defensible borders, push it back to the vulnerable 1967 lines, and redivide the heart of Jerusalem as well. Washington has also praised the Saudi plan, seeming to have forgotten the declarations of at least a half-dozen U.S. secretaries of state.

Israel should always seek peace when possible, but it has no legal or historical obligation to endanger itself by adopting the map that the plan suggests. Once, statesmen understood that the only way to create a stable peace is by crafting a peace that can be defended. It was the lesson of those who sat on the U.N. Security Council in 1967. It clearly needs to be relearned again today.

Mr. Gold, the former ambassador of Israel to the United Nations, is the author of “The Fight for Jerusalem” (2007). He is the president of the Jerusalem Center for Public Affairs.