Pretend for a moment it was Israel, rather than the United States, that was hit by the attacks of Sept. 11. Would the world now be waging the same war against terrorism, with the same arsenal of weapons and the same intensity?

Like you, I don't know the answer. But I do know, or at least suspect, that if Tel Aviv had been targeted, the Israelis would have retaliated against the murderers and their supporters with such overwhelming fury and force that the entire Middle East would now be engulfed in war. They wouldn't have asked anybody's permission, and they wouldn't have worried much about coalitions.

The Israelis have had to deal with radical Islam for decades. Their safety -- indeed, their very survival as a nation -- depends on their ability to keep this enemy reasonably at bay. The United States has blanched at their tough tactics at times, and Jerusalem normally will go only so far when Washington expresses its displeasure, but they've earned their reputation for toughness the hard way.

This tiny outpost of Western democracy wouldn't respond to a bloody massacre on the scale of Sept. 11 with benefit concerts. They would launch an immediate and explosive response, directed not only at the terrorists but at the "good neighbors" who had provided the terrorists with resources and safe harbor: Iraq, the Palestinian Authority (the killers formerly known as the PLO), Syria, the Syrian puppet regime in Lebanon, and any other neighbors that were complicit in the attack.

Such a nightmare scenario could lead easily to World War III. So we need to ask ourselves seriously: What would the United States do if Tel Aviv became the target of a devastating attack killing thousands, or tens of thousands, and the Israelis decided, once and for all, to wage a war to end all wars in the Middle East?

Make no mistake: Some of Israel's neighbors are capable, both psychologically and militarily, of attempting such an attack. Because of Israel's military strength and the fact that the country's been on high alert against terrorism almost from Day One, the most likely means of launching such a doomsday attack would be a barrage of ballistic missiles.

Yasser Arafat's terrorist pals in Hamas, Hezbollah and the Popular Front for the Liberation of Palestine don't have such missiles yet. But Syria, Iran and Iraq do. Syria has an estimated 600 ballistic missiles, putting not only Israel, but parts of Russia and Turkey within range. Iran has some 700 Scud-type missiles and is developing a long-range missile that could reach Europe and much of the United States.

And though his missile program was dealt a setback during the Gulf War, Iraq's Saddam Hussein is thought to have as many as 150 missiles at his disposal.

Fortunately, Israel has at least a partial remedy. Unlike the United States, which would be defenseless against a ballistic missile launched from another country -- or even from a ship or barge that positions itself several hundred miles off shore -- Israel (with American help) has built an anti-missile missile, the Arrow. So the Israelis could intercept at least some, if not most, of an incoming missile volley.

When you consider the harm that could be caused by even one missile armed with a nuclear warhead or a deadly biological or chemical agent, the need for such a missile defense becomes clear.

As we learned Sept. 11, instant mass murder is no longer unthinkable. In fact, certain Muslim fanatics apparently consider such attacks against the western "infidel" (meaning you and me, as well as Israel) a courageous act of religious purification. You've already seen the pictures of Osama bin Laden's followers and supporters rioting in the streets. Pity the civilized world if they ever take control of a country with a nuclear arsenal, such as Pakistan, or gain operational control of even a single nuclear missile, or a battery of missiles.

The United States has buried its head in the sand for too long. Despite the continued talk of unity, some in Washington -- for whatever reason -- still want to tie up or slow down America's missile defense program.

Let's get serious. We don't need more debates. We need to put the missile-defense program on emergency fast forward, and get the job done as soon as humanly possible -- before the next "unthinkable" horror becomes our next national nightmare.

Edwin Feulner is president of The Heritage Foundation a Washington-based public policy research institute.