Walking briskly through the American Capitol last week with several Israeli guests, I paused to consider Emanuel Gottlieb Leutze’s brilliant rendering of American history, “Westward the Course of Empire Takes Its Way.” The 1861 oil study shows a wagon train of settlers descending from the Sierra Nevada range to the Golden Gate near what would become San Francisco. “Look,” I told my guests, “a painting about Israel leaving the Gaza Strip.” Only upon close study does it become clear that the push is toward the Pacific, not back toward the Atlantic. In America, the westward direction is assumed.

Thinking about the two historical movements – American settlers spreading out across the North American continent and Israeli settlers moving into the West Bank, Golan Heights, and Gaza Strip – I was struck by the role of the frontier in the development of both nations. Indeed, one of the least remarked aspects of Israel’s impending withdrawal from the Gaza Strip and northern West Bank is that it marks the beginning of the closing of Israel’s frontier.

Arguing for a “frontier thesis” in the context of the Israeli-Palestinian Arab dispute does not render a moral judgment. Native Americans no doubt have their own, vastly different versions of the story of America’s westward expansion. Palestinian Arabs see Israel’s war of independence (Milhemet ha’Shichrur) as Al-Nakba, The Catastrophe. For Leutze’s “Westward,” there is the counterpoint of Robert Lindneux’s “Trail of Tears.”

American expansion was justified under the slogan of “manifest destiny,” a term first coined by John L. O’ Sullivan in an editorial in the December 27, 1845, number of the New York Morning News. Writing about America’s claim to Oregon, then in dispute with Britain, O’ Sullivan wrote that America has “the right of our manifest destiny to overspread and to possess the whole of the continent which Providence has given us for the development of the great experiment of liberty and federated self-government entrusted to us.”

Israeli expansion into the territories has been justified narrowly on security grounds and more broadly as a natural extension of the Jewish people’s return to their ancestral homeland, a process that, its advocates argue, is as legitimate now as it was during the events that led to the 1949 cease-fire and the now retroactively sanctified 1967 borders. They say, “If Israel has no right to Gush Katif,” the bloc of Jewish settlements in Gaza to be evacuated, “it has no right to Tel Aviv,” Israel’s largest city which began as a Jewish settlement north of the Arab-majority city of Yafo. There is also a messianic version (among some Jewish and Christian groups) that sees the retention of the territories under Jewish control as a necessary precondition for messianic redemption.

The real distinction to be drawn is that the inexhaustible supply of European immigration made the Americans invincible against the Native Americans, who were outnumbered and outgunned. By contrast, although Israel has technological and military superiority, the Arabs generally, and the Palestinian Arabs among them, have an inexhaustible supply of people ready to fight and die, while the Israelis worry about demographics and declining birth rates.

Moreover, the times have changed. The zeitgeist that applauded the march of Western civilization over aboriginals has been swept away by our postcolonial times: Nowadays, Europeans would sympathize with the Indians, not the Cowboys.

Nevertheless, and mutatis mutandis, the severe shock to Israeli society set in motion by Prime Minister Sharon’s disengagement is not only, or even mainly, based on a fault line between hawks and doves, religious and secular, or messianists and pessimists. The disengagement marks the first time that the Israeli state has turned against the frontier and the frontiersmen who have, until now, disproportionately influenced the direction and substance of national policy, certainly with regard to settlement (and therefore policy toward the Palestinian Arabs).

In the initial stages of its development, Israeli settlement was characterized by the kibbutz and moshav movements of the left. Most Israelis lived urban (and today suburban) lives, but the nation was guided, certainly in the matter of its borders, by the “pioneers in front of the camp.” Following the 1967 war, the role of the kibbutz and moshav movements receded, though they have not entirely vanished, while the religious Zionist settlers captured the settlement flag. Though not socialist-oriented, these settlers shared with their left-wing predecessors a contempt for the urban life of the emerging Israeli middle class and portrayed their own lives along the frontier, to borrow from the American historian Frederick Jackson Turner, as “the meeting point between savagery and civilization.”

Three years before Turner presented his thesis at an 1893 gathering of historians, the Census Bureau had announced the disappearance of a contiguous frontier line, which Turner saw as the “closing of the frontier.” Turner argued that having an open frontier had allowed Americans to experience over and over again “primitive conditions on a continually advancing frontier line,” reliving the story of the original settlers.

He wrote that this shaped the American character, because “the frontier is the line of most rapid Americanization,” inducing “that coarseness and strength, combined with acuteness and acquisitiveness; that practical inventive turn of mind, quick to find expedients; that masterful grasp of material things … that restless, nervous energy; that dominant individualism.”

Turner could have been writing of Israel when he argued that “the existence of an area of free land, its continuous recession, and the advance of American settlement westward explain American development. “The directions were reversed, with Israeli expanding every way but westward. Otherwise, the consequences were similar: anti-urbanism, strong individualism mixed with ardent nationalism, and a sense of meta-historic entitlement. With every new constellation of hilltop caravans, Israeli settlers could over and over again relive the earliest days of Zionist pioneering settlement. From a cultural perspective, therefore, the disengagement is a victory of the urban middle class over the frontiersmen, and involves a repudiation of, indeed a withdrawal from, the frontier itself.

Turner concluded, “And now, four centuries from the discovery of America, at the end of a hundred years of life under the Constitution, the frontier has gone, and with its going has closed the first period of American history.” That changed everything; and the disengagement too promises to change everything that has gone before, during the first period of Israeli history. Such seismic changes induce fierce resistance, even national trauma, points that need to be understood even by those who feel Israeli withdrawals from the territories which fell into its hands as a result of the 1967 war are long overdue.

Mr. Twersky is a contributing editor of The New York Sun.