With the end of the Bush presidency in sight, the situation in almost all areas of domestic and foreign policy is worse than it was. On Iraq, however, we in Israel owe him an enormous debt. Indeed, if I were religious, I would find a passage showing that the invasion was the realization of a biblical prophecy, the act of one of the great deliverers of the Jewish people. Bush went to war for reasons almost entirely unrelated to Israel, but in doing so, may have saved us from an apocalypse. Consider two alternative scenarios. The terrible scenario: Fast forward, summer 2011. After five years of tenuous quiet, a vastly strengthened Hizbullah decides to fire rockets at Israel, which counterattacks, determined to end the threat once and for all. Iran, having successfully thwarted all diplomatic efforts, announces that it has "the bomb," hinting at Israel's destruction. Syria, Hizbullah's other patron and an Iranian ally, concentrates forces. The US goes on nuclear alert. The Security Council convenes in emergency session. Oil prices... The even worse scenario: All of the above, but the US never invaded Iraq and it is still presumed to retain a residual WMD capability; Libya never chose, largely in response to Iraq, to dismantle its nuclear program. They too now join the fray. Today we know that Saddam Hussein did not have WMD, but to those of us in the US and Israeli governments at the time, who were sincerely convinced that he retained a residual program, it was an analytical reality. We were very wrong, but we were not irresponsible, nor malevolent. Israel, in case you have forgotten, took the threat very seriously and distributed gas masks, deployed forces and asked the US for antimissile defenses. PRESIDENT BARACK Obama will come into office at a time when Iraq increasingly looks like a moderate success. No, Iraq will never be a Jeffersonian democracy, but an Arab one; the only one. The security situation is immeasurably better; Iraq's economy is rebounding. Success is not yet irreversible, but it now appears that only the US can snatch defeat from the jaws of victory, by withdrawing precipitously. If the US withdraws prematurely, Iraq may still deteriorate into ever worsening violence and splinter into its component parts. Turkey may then invade Kurdistan, whose possible independence it views as a threat to its territorial integrity. Iran will become not only a primary player in Iraq, as it already is, but the primary one, possibly even annexing parts of Iraq outright. The Saudis, already threatened by rising Shi'ite influence in the region, are petrified by the thought of a nuclear Iran right on their border. Jordan, with an Iranian entity on its border, already inundated by Iraqi refugees, would fear an existential threat to its stability. If the US can still be driven out of Iraq, even now, when at least partial success is within sight, the Islamist fundamentalists and other dark forces in the region, will have won. Iran will end up the big victor, the regional hegemon, whose ambitions, nuclear and otherwise, will become unstoppable. There will simply be no one to prevent the triumphant radicals from using terror, WMD, subversion and religious fanaticism to pursue their aims. As things look now, there is a very real possibility that Iran will soon announce an operational nuclear capability. The current worst-case scenarios give Iran sufficient fissile material for a first bomb sometime in 2009 or early 2010. Even if the timetable proves to be longer, the Iranian issue will be on the new president's plate from scratch. A NUCLEAR Iran is a dire threat, not just for Israel, but for other countries in the region. In response, Egypt, Saudi Arabia, Turkey and others have already announced new "civil" nuclear programs, which have a nasty tendency to morph into military ones. It is unclear that either Israel or even the US can effectively cope with a nuclear Iran, but a multi-nuclear Middle East now looms, an as yet uncharted danger with which no known strategy is equipped to deal. In the yet more radicalized Middle East that will follow a premature withdrawal, we can abandon any hopes for even limited reform in the region and for any change in the root causes of its ills. The regimes will heighten oppression to retain power, the radical camp will be emboldened, the prospects for peace even dimmer. If the Mideast looks bad today, just wait. More fundamentally, the harsh reality is that the US cannot truly disengage from Iraq and the Middle East, no matter how much it wants to. The region's deep seated and violent ills will follow it back home regardless. The extremists' hatred of the US did not begin with Iraq and will not end when it leaves. Unfortunately, all human societies, including the "international community," require policemen to maintain order. The US may not wish to bear this burden, but there is no one else. George W. Bush had it right when he contended that failure in Iraq is unacceptable. We will see if Obama has similar courage. The writer is a senior fellow at Harvard's Kennedy School of Government, Belfer Center.