Editor's note: David Frum writes a weekly column for CNN.com. A special assistant to President George W. Bush from 2001 to 2002, he is the author of six books, including "Comeback: Conservatism That Can Win Again," and is the editor of FrumForum.
(CNN) -- It's not just the geography. It's the topography.
When you look at maps of the Middle East, you see at once that pre-1967 Israel was very narrow -- just nine miles across. The Dallas-Fort Worth International Airport stretches wider than that.
But what you do not so easily see is that pre-1967 Israel was low as well as narrow. Israel emerged from the 1948-49 Arab invasions holding the coastal plain along the Mediterranean. The invaders grabbed and held the highlands between the plain and the Jordan River Valley.
Those highlands rise almost 3,000 feet above the coastal plain. Whoever controls them can shower missiles and rockets upon Israel's cities and factories -- with much greater accuracy and lethality and much less vulnerability to retaliation from ground forces than a rocket fired from level ground.
Israel experienced such a barrage in the three years leading up to the 2008 Gaza war. Thousands of rockets were fired from Gaza into southwestern Israel. Southwestern Israel is relatively lightly settled, so most of the rockets exploded without killing anyone. Even so, Israel suffered 16 killed and dozens wounded by rocket fire between 2005 and 2008. Had those rockets been fired into central Tel Aviv -- or against the runways of Ben Gurion Airport -- they would have inflicted horrific human and economic damage.
Holding the highlands matters for external as well as internal security. The West Bank boundary with Jordan extends for almost 100 kilometers (more than 60 miles). Yet in all that length, Israeli military experts assess that there only three routes across which an armed force can travel. Whoever holds those controls land access to Israel from the east.
In his speech Sunday to the American Israel Public Affairs Committee, President Barack Obama emphasized that Israeli forces would be excluded from the territory of a future Palestinian state in the West Bank. He called for a phased but complete withdrawal. After that point, it would be up to the forces of the Palestinian state to protect Israeli cities from West Bank rocket fire and to defend the Jordan River crossings.
The president has endorsed the concept that Israel defend itself, by itself. Yet Obama's statements on borders, if implemented, would put the decisive power over Israel self-defense into the hands of a new state that will at best not be very capable of doing the job -- and at worst outright hostile to Israel.
To achieve a lasting peace in the region, Palestinians should be encouraged to see that their hopes for self-rule depend on answering Israel's important security questions.
The president's approach this past week is having the opposite effect. Palestinians have refused to negotiate with Israel since Obama's election. Until now the excuse has been that they won't talk until Israel stops all home-building, not only in the West Bank, where Israel had once accepted a freeze, but in Jerusalem, too. On Sunday, the chief Palestinian negotiator, Saeb Erakat, added a second precondition: no talks until Israel publicly accepts the Obama statement on borders.
But Israel won't agree on borders until those other security considerations have been answered. To prejudge the border issue -- and only the border issue -- makes agreement harder, not easier.
Obama made a serious mistake in his Thursday speech by committing himself in advance in this way. He dug in deeper in his speech to AIPAC on Sunday. The president's rash attempt to push the peace process forward has instead rewarded two years of Palestinian intransigence with an excuse for more intransigence still. The president himself by his unwise words pushed further away the outcome he seeks to advance.
The opinions expressed in this commentary are solely those of David Frum.