Yesterday, I took issue with the Union for Reform Judaism for condemning planned Israeli construction in the West Bank’s E-1 region. Many liberal American Jews would doubtless respond that they don’t object to E-1 remaining Israeli under an Israeli-Palestinian agreement; they merely object to building there before such an agreement exists. That, after all, is precisely what Ehud Olmert said last week when asked how he could condemn the Netanyahu government for doing something he himself supported as prime minister.

Unfortunately, this response betrays a serious lack of understanding of how the “peace process” actually works. First, as I noted yesterday, insisting that Israeli construction is an “obstacle to peace” even in areas that every proposed agreement has assigned to Israel merely encourages Palestinian intransigence by feeding their fantasies that the world will someday pressure Israel into withdrawing to the 1967 lines. Equally important, however, is that in a world where Israeli security concerns are routinely dismissed as unimportant, construction has proven the only effective means of ensuring Israel’s retention of areas it deems vital to its security.

In theory, construction shouldn’t be necessary to stake Israel’s claim, because the world has already recognized it: UN Security Council Resolution 242, still officially the defining document of the peace process, explicitly recognized Israel’s right to obtain “secure” borders by retaining some of the territory it captured in 1967, since, as then-U.S. Ambassador to the UN Arthur Goldberg explained, “Israel’s prior frontiers had proved to be notably insecure.”

But in practice, the only parts of the West Bank that successive peace plans have envisaged Israel retaining are the ones where there are just too many Jews to easily remove. As former President George W. Bush put it in his 2004 letter to then-Prime Minister Ariel Sharon, “In light of new realities on the ground, including already existing major Israeli populations centers, it is unrealistic to expect that the outcome of final status negotiations will be a full and complete return to the armistice lines of 1949, and all previous efforts to negotiate a two-state solution have reached the same conclusion.”

In contrast, the world has generally dismissed Israeli demands to keep sparsely settled areas, even when they are equally vital for security. For instance, all Israeli governments have considered military control over the Jordan Valley essential for security, but even Washington hasn’t backed this demand. And the European Union is much worse: It officially views the entire West Bank as occupied Palestinian territory to which Israel has no claim whatsoever unless the Palestinians allow it.

For this reason, Israel should long since have built in E-1–an area every Israeli premier has deemed vital for security–rather than leaving it vacant at the urging of successive U.S. administrations. But the issue received new urgency after the UN overwhelmingly recognized a Palestinian state last month “on the Palestinian territory occupied since 1967.” With virtually the entire world having just declared that Israel has no right to any part of the West Bank, it has become imperative for Israel to strengthen its claim via the only means that has ever proven effective: by building.

The question now is whether Israel will actually do so, or whether its government will once again sacrifice the country’s long-term security needs on the altar of global opposition.