THE LATEST ROUND of estimates of the cost of a potential Iraq war has rekindled the mistaken suggestions that bubbled up last September. Then, Lawrence B. Lindsey, President Bush's economic adviser, guessed that a war might cost $100 billion to $200 billion; lately various think-tankers and economists have scribbled on the backs of envelopes and produced similar numbers. This exercise has prompted debate about whether this is a lot or a little. It's only 1 percent or 2 percent of U.S. annual output, not much compared with the 15 percent of GDP that the nation devoted to the Korean War or 12 percent for the Vietnam War. On the other hand, it can sound like a lot if you use other yardsticks. War could double the past year's $159 billion federal budget deficit, for example.

This debate is futile because the case for war depends on its strategic consequences, not on its price tag. You wouldn't argue, for example, that the Vietnam War was a success because it cost less (as a share of GDP) than the Korean War, or that World War II was a failure because it cost much more than both. The historical judgment on all these conflicts depends, rightly, on whether they increased the security on which liberty and economic progress depend. The same is true of an Iraq war. If you think Saddam Hussein poses no threat, it would be crazy to wage war against him, no matter what the price tag. But if you think he may one day use weapons of mass destruction to blackmail his oil-producing neighbors and threaten American security -- and if you think, crucially, that the Bush administration will be committed to the long task of creating a better regime after the war's done -- then $200 billion or even $400 billion would be worthwhile.

Yet there are two ways in which the talk of war costs may be constructive. First, it underlines the case for finding allies: In the Persian Gulf War, the United States successfully recouped all but $7 billion of the costs from other nations. Second, it may help focus attention on this nation's neglected long-term finances. For although the strategic consequences of an Iraq war far outweigh the cost issues, these costs will have to be met somehow. Moreover, the Iraq war is only one part of Mr. Bush's determination to preempt security threats, which encompasses an ambitious homeland security effort, an expanded defense budget and a willingness to slow the gears of trade and international finance by increasing the burden of inspection by customs officials and terrorism-finance task forces.

Mr. Bush is failing particularly to face up to the second point. He has failed, for example, to propose a war tax to pay for the cost of containing terrorism and Iraq. He argues, to the contrary, that taxes should be cut, even beyond the already unaffordable tax cut of last year. And he behaves this way despite an expert consensus that the budget will plunge into sustained deficits once the baby boomers start to retire, even before you take account of the increased security costs that now seem likely. A president who believes in preempting problems ought to show more courage.