Detlev Mehlis is a German prosecutor who, in the name of the world, is accusing Syrians of a crime against the Lebanese.

The United Nations, with Mehlis's preliminary report on the murder of Rafiq Hariri, the former prime minister of Lebanon, is plunging headlong into unexplored territory -- where the relatively novel tool of international sleuthing confronts the ancient Machiavellian tool of state-sponsored murder. The report, which Mehlis produced on behalf of the U.N. International Independent Investigation Commission, puts the world body neck-deep in the ongoing crisis between Syria and the United States. Even more importantly, the report puts the U.N. in the business of stigmatizing and punishing individuals for a political crime.

Mehlis's tough 54-page preliminary report, which implicates senior Syrian officials for assassinating Hariri in Beirut on Feb. 14 in order to maintain Syrian influence over Lebanon, makes it hard for the U.N. Security Council not to take action against Syria. Even before Mehlis concludes his investigation on Dec. 15, he has stingingly accused Syria not just of murder but also of stonewalling his inquiry. Saad Hariri, the murdered prime minister's son, has even called for a special international tribunal. More likely, the Security Council will authorize a variety of sanctions.

Previously, the U.N.'s war crimes tribunals for the former Yugoslavia and Rwanda focused on obtaining indictments of top-ranked government officials allegedly involved in murder on a massive scale -- the death of hundreds of thousands of civilians. But now the U.N. is effectively extending its jurisdiction to include the death of a single person, Hariri, as well as 22 others killed by the Beirut car bomb. While genocide is an extraordinary and rare event, political assassination and injury are more commonplace. Witness the August assassination of Sri Lankan Foreign Minister Lakshman Kadirgamar and the March 2003 murder of Serbian Prime Minister Zoran Djindjic; and recent plots in Afghanistan, Pakistan, Uganda, Iraq, Ukraine and elsewhere. Interstate assassination is rarer, but there is a particularly salient example: a 1993 Iraqi intelligence plot to assassinate former president George H.W. Bush, prompting the current president to call Saddam Hussein "a guy that tried to kill my dad." So the Mehlis investigation into Hariri's murder, if it becomes any kind of precedent, would radically widen the scope of U.N. intrusiveness.

When national institutions cannot function, it makes sense to turn to international ones. Slobodan Milosevic, the toppled Yugoslav strongman, had to face justice before a U.N. tribunal because Serbian domestic courts were poised to prosecute him mostly for corruption, not for his atrocities against non-Serb civilians in Bosnia, Croatia and Kosovo. The International Criminal Court (ICC), which the U.S. government opposes, only gets jurisdiction over a war crimes case if national courts cannot or will not handle it properly. In the case of the Hariri assassination, it is clear that the Syrian justice system would never look honestly at who killed him and that Lebanese prosecutors would be too frightened both of Syria and of domestic ethnic unrest to dig deeply. The Arab League is not in the habit of hounding its member governments about human rights. So that left the international sphere.

This does not mean that the U.N. will now jump in after every assassination anywhere -- any more than the U.N. jumps in to prosecute every war crime. Lebanon is a special case: a high-profile plot in a country already under the Security Council's watchful eye, and with a government that agreed to an international investigation. But if there is an assassination in a country that the Security Council is already paying close attention to -- such as Burundi, Sierra Leone or Liberia -- the U.N. could come under pressure to launch a Mehlis-style investigation. Michael Doyle, a former U.N. assistant secretary-general, says, "Mehlis is sort of acting like a district attorney who is presenting evidence to a grand jury, although the Security Council is not exactly a grand jury."

How far can the U.N. go? The Mehlis investigation provides crucial international legitimacy and credibility to accusations against Syrian President Bashar Assad's regime. But punishing the guilty, whether in ex-Yugoslavia or Cambodia or Syria, requires not just legal legitimacy but raw-knuckle international politics.

A demand for international legalism does not mean that there will be a supply. The U.N. faces sharp political limits. International institutions and international law are more likely to have teeth when they serve some powerful political constituency. On a recent trip to Bogota, I was surprised to hear Francisco Santos Calderon, Colombia's vice president, explain that his government benefited from the ICC because Colombian paramilitary groups were more afraid of an international trial than a Colombian one. Here was an unexpected way in which having an international legal option in its toolkit gave a government more room for creative diplomacy, not less.

More bluntly, international machinery works only as well as the great powers will allow it to. The League of Nations was destroyed by fascist hostility and democratic appeasement and isolationism. During the Cold War, U.S.-Soviet tensions regularly deadlocked the Security Council. In the run-up to the Iraq war, the Bush administration failed to win over France and Germany. The reluctance of Russia to punish Iran for its nuclear ambitions has made it harder for the International Atomic Energy Agency to refer a case against Iran to the Security Council. In the Hariri case, Algeria is likely to try to shield Syria, and China and Russia might drag their heels out of fear of new precedents that impinge on their own sovereignty. If China or Russia cast a veto, then the window that Mehlis opened will be slammed shut.

But the Mehlis report has provided a rare post-Clinton moment of Western harmony. The Bush administration's aversion to going the U.N. route is well documented. Bob Woodward, in his book "Plan of Attack" on the preparations for the Iraq war, wrote: "[Vice President] Cheney harangued about the United Nations. Going to the U.N. would invite a never-ending process of debate, compromise and delay."

But at least for now, Jean-Marc de la Sabliere, the French ambassador to the U.N., is on the same page as John R. Bolton, his American counterpart. Britain is on board, too. Secretary of State Condoleezza Rice is lobbying her fellow foreign ministers hard to take tough steps against Syria in a vote scheduled for Monday. "We've always believed the U.N. can make a useful contribution to international peace and security," says a senior State Department official. "The Mehlis report and the Security Council's consideration of it is not a dramatic change in direction for the United States."

While the White House fiercely opposes the ICC, it sometimes finds international institutions convenient. It has supported war crimes tribunals in ex-Yugoslavia, Rwanda and Sierra Leone -- where American soldiers and leaders are not exposed to prosecution. And the White House did not block the Security Council's referral of atrocities in Darfur, Sudan, to the ICC. The United States continues to back U.N. multilateral peacekeeping missions. Instead of blanket hostility, the Hariri case shows the Bush administration using the U.N. a la carte, picking and choosing the functions it likes.

For instance, one of the toughest Bush administration criticisms of the ICC was that its prosecutor would be unaccountable and thus could launch politically motivated investigations. But even though the Bush administration did not have direct control over Mehlis, his inquiry has proved a political bonanza for the foes of the Syrian dictatorship inside the White House.

Immediately after Hariri's assassination, the Bush administration withdrew the U.S. ambassador to Syria, but Rice said she was not laying blame and called for an investigation. Now the U.N. report's tough findings give credible cover to what could otherwise be more easily dismissed as an anti-Syrian vendetta in Washington. Bolton, who in 1999 warned of the potential menace of "a zealous Global Chief Prosecutor," is today conspicuously fulsome in his praise of Mehlis's professionalism and integrity as a prosecutor.

Will the Mehlis investigation be the first of many? At a minimum, it has already proved its usefulness. By replacing American partiality with an ostensible U.N. impartiality, it has provided crucial legitimacy to what otherwise would be mere accusations. It is too soon to call this a precedent. One such investigation could be a fluke. The most that could be said is that the creation of the ex-Yugoslavia war crimes tribunal paved the way for international courts in Rwanda and Sierra Leone, as well as the ICC. Useful tools have a way of getting picked up.

Gary Bass, an associate professor of politics and international affairs at Princeton, is the author of "Stay the Hand of Vengeance: The Politics of War Crimes Tribunals" (Princeton University Press). He is writing a book on humanitarian intervention.

Uncharted territory: Syria's ambassador addresses the United Nations, where the modern art of international sleuthing is being used to investigate alleged state-sponsored assassination. Undeterred: U.N. investigator Detlev Mehlis delivered a report that will not be ignored.