Should it come, the war will not be about cheap oil -- but about preventing oil power from being tragically misused. We have not sent nearly 400,000 Americans to the Persian Gulf to defend crude at $18 a barrel. They are there to keep Saddam Hussein from controlling two-thirds of global oil reserves and from using that control to blackmail the industrial world and make Iraq a nuclear power.

The case for fighting Saddam now is, simply put, that he will be harder to fight later. And should last-minute diplomacy avert war, the success will be temporary unless it leads to effective measures to check Saddam's military strength and provide security in the Gulf.

Because war is so awful, we must be clear that our purpose is worthy. The stakes are more important than a few cents on gas prices or restoring the emir of Kuwait. Nor are we defending opposition to all aggression, no matter how brutal. A president who did that would be "impeached after the first crisis in which there were significant U.S. casualties," as Foreign Policy editor Charles William Maynes says.

What we are doing is maintaining access to the oil on which the world depends. The aim involves, as most Americans intuitively grasp, a high-minded pragmatism. It is an effort to sustain the well-being of billions of people, ourselves included. We import half our oil, but even self-sufficiency would not fully insulate us from turmoil in the Gulf. Our prosperity is heavily tied to countries that inevitably need the Gulf's oil.

If price were all that mattered, we could rely on the market. Even for producers, excessively high prices don't maximize profits. They drive away buyers by promoting conservation and inducing new oil production. If Iraq controlled most Persian Gulf oil, it would maximize its revenues at a price of about $25 a barrel, estimates economist William Niskanen of the Cato Institute. Significantly, that's lower than today's price.

The trouble is that Saddam isn't a profit maximizer. He doesn't merely want to rip us off: an infuriating but relatively mild indignity. Saddam's ambitions are "to dominate the Arab world . . . and ultimately to turn Greater Iraq into a global military power," as Daniel Yergin writes in his compelling history of oil, "The Prize."

To cede him control of the Gulf would compound his potential for havoc. Would he dole out oil at favorable prices to countries that delivered nuclear or military hardware? Would he foment and finance more terrorism? With his treasury replenished, might he simply stop Persian Gulf shipments for a few months to win Western support for his political agenda? Might he decide that oil at $60 a barrel suited his politics, even though it might not be "profit maximizing"?

No one can say exactly. But a stronger Saddam would not be less threatening. Although we may not know much about him, we know enough. He relishes war and uses every weapon in his arsenal. ("Weakness doesn't ensure achieving the objectives required by a leader," he said last year.) What else do we need to know? A stronger Saddam would guarantee new wars in the Mideast: with Israel, the only question may be who would strike first. But Syria, Iran and Saudi Arabia would also be obvious targets.

Just because the consequences aren't predictable doesn't mean we should wait until they occur. A future war or political chaos could involve a huge loss of life (probably 500,000 or more died in the Iran-Iraq War) and cripple oil production. Even now, Iran's output is only about half its level before the shah fell in 1979. And in the 1990s, the world will depend more on oil from the Gulf, says Bernard Picchi of Salomon Brothers. Production will slip in some big fields outside the region: Prudhoe Bay, the North Sea and Mexico. Meanwhile, demand from developing countries is rising.

For the United States, the cost of fighting now would be huge. In fiscal 1991 the price of maintaining U.S. forces in the Gulf, without fighting, is put at $30 billion. A shooting war could inflate that figure by tens of billions. On a daily basis, some analysts think the cost could range between $500 million and $1 billion. (A caveat: this doesn't mean a yearlong war would cost $365 billion. A war probably wouldn't last that long, and if it did, the intensity of early fighting -- and costs -- would not.)

This crisis, however, cannot be ended by either a late diplomatic settlement or a decisive military victory. There would be a sigh of relief and probably good economic news. Oil prices might drop. That would act like a tax cut that might cushion the recession, says Philip Verleger Jr. of the Institute for International Economics. Yet, the basic problem would remain of bringing stability to an unstable region.

At best, this will be difficult. No one can foresee the course of war or its aftermath. The passions of Arab nationalism have been stirred once again. The legitimacy of the rich Gulf states has been shaken, because so many Arabs are so poor. The Palestinian issue festers. Our position in the region founders on the contradiction between our oil interests and our support for Israel. We have differences with the Europeans and Japanese. Our capacity to provide permanent security is unclear. Too large a military presence might abet political instability. Too little might not work.

The true cost of war will be measured by its dead. The issue today is whether a horrible war now will reduce the risk of a more horrible war later. But this cannot comfort a grieving parent or child. Unless we can win the peace, war's sacrifices will be in vain.