THE FIRST presidential campaign debate, scheduled for tomorrow, has proved to be well timed. The campaign's focus has finally shifted from Vietnam to the war this country is now fighting, and in the past 10 days President Bush and Sen. John F. Kerry have staked out what sound like distinctly different positions on it. That creates plenty of opportunity for good questioning at a forum that is to be dedicated to foreign affairs. Above all, there is an opportunity to learn whether the rhetorical differences between the candidates would lead to genuinely different courses of action in Iraq and elsewhere -- something that for now isn't very clear.

The necessary prelude to that practical focus, it seems to us, is a large and unresolved question of principle: What is the war on terrorism? Shortly after Sept. 11, 2001, there appeared to be a consensus that this country faced a long and complex struggle against extremists who had chosen to attack the United States, as well as against states that supported them and might supply them with weapons of mass destruction. Three years later it's not so clear. Mr. Bush still describes that kind of broad war and places Iraq at the center of it; Mr. Kerry says Iraq was a diversion from the "real enemy," al Qaeda.

The crucial question here is not whether there were links between Iraq and al Qaeda, but whether the war should be understood as extending beyond that terrorist group and others associated with it. Does it include confronting states that sponsor Islamic terrorism, such as Iran and Syria? Does victory require a political transformation of the Middle East, or a solution to the Arab-Israeli conflict, or more intensive nation-building in failed states? Mr. Kerry's recent speeches suggest a narrower focus; is it right that more concentration on al Qaeda and Afghanistan would make the United States safer than a concurrent campaign against Iranian-sponsored terrorism or for free elections in Egypt? If Osama bin Laden were captured and al Qaeda were destroyed, would the war be over?

Defining the war on terrorism helps clarify where America's interests now lie in Iraq. Despite their differences, Mr. Bush and Mr. Kerry seem to advocate the same basic strategy, which is the construction of a national Iraqi government via elections and the rapid training of security forces to defend it. That raises an obvious question: Would either candidate consider one of the other solutions that have been proposed for Iraq -- which include real or de facto partition of the country or the setting of a firm and early date for the departure of American troops? Does Mr. Kerry's view of the war as a "diversion" mean that he views either success or failure there as not critical to the struggle against terrorism? Must U.S. forces act to eliminate foreign terrorists and their bases, or is that a job for Iraqis?

Although Iraq may dominate foreign policy in the next several years, the biggest presidential decisions could lie elsewhere, from Taiwan to Pakistan. One looms now in Sudan: Would either Mr. Kerry or Mr. Bush use U.S. troops to stop genocide in Darfur, or do they rule out such a mission? Next will come Iran and North Korea. Mr. Bush has vowed that the United States will not "tolerate" Iran's acquisition of nuclear weapons and aims at dismantling North Korea's presumed arsenal. Mr. Kerry doesn't seem to differ on these goals -- but what do they mean? Both the president and his challenger conveniently suppose that diplomacy will somehow break what looks like the determination of both countries to become nuclear powers. But what if diplomacy doesn't work? Voters will judge Mr. Bush and Mr. Kerry on their prescriptions for the battles the country now fights; but in this election, they must also think about those that may be just beyond the horizon.