It takes two to stop shooting. Israel will most likely agree to do so, sometime over the coming weeks. That shifts the focus to Hizbullah. Will it withdraw, if only to save its fire for another day? If so, Secretary of State Condoleezza Rice and European leaders, who kicked off what promise to be long and difficult negotiations today in Rome, might eventually be able to win a ceasefire and cobble together a peacekeeping force for southern Lebanon. The key question will be its mandate. Israel insists that it include enforcing United Nations Resolution 1599 and disarming Hizbullah—setting up any international mission for trouble. Almost certainly, Hizbullah would shoot back.

This highlights an inescapable fact about humanitarian interventions. They succeed only when circumstances are right. I say this from personal experience in other arenas where the international community stepped in—Bosnia and Kosovo, where I served for two years with the United Nations mission that rolled in to keep the peace after NATO’s 1999 bombing campaign.

Kosovo can be counted a success for two reasons. First, the principal aggressors in the war—the Serbian Army commanded from Belgrade—were driven out. Second, those who remained wanted peace. To be sure, a very small minority of firebrands targeted Serbs remaining in the province. But most people wanted merely to rebuild their homes, some 80 percent of which had been destroyed in the war, and get on with their lives.

I remember, one day, an international forensics team going up into the hills above Pristina, the Kosovar capital, to investigate claims of a mass killing. As the team dug in an impossibly green and placid hillside, a farmer sat under a tree, calmly smoking as he related how his family (among others in his village) had been killed by Serb neighbors from just down the hill. He gestured, almost casually, to a white-wattled cottage less than a quarter mile away, smoke rising from its chimney. “Him,” said the man. “Mirko.”

As he told it, Serb villagers had come with special army forces, little more than a year before. The local Albanian men hid in the surrounding forest and waited, thinking no harm would befall their women and children. Instead, the farmer said, the Serbs shot them all, then burned their houses. He and a brother watched as his grandmother was dragged out of the house and shot through the neck, calling all the while to her neighbor and former friend: “No, Mirko. No!”

The diggers soon unearthed her body, buried where the farmer had said—a gelatinous blackened mass, but not so decomposed as to hide the bullet wound to her throat. Why hadn’t he shot Mirko out of revenge? After all, the man who helped murder his family lived just over there, tending his crops and pigs. “Because,” he said, “I was waiting for you. For justice.” No one had the heart to tell the man that it was unlikely Mirko would ever face justice. After all, he was but a cog in the Yugoslav killing machine, and far greater war criminals than he would go unpunished. Yet that’s not what counts. The important point is that the farmer waited. He did not take justice into his own hands, nor did he vent his hatred of Mirko upon all Serbs.

The other Balkan war, in Bosnia, was almost completely different. When the forces of UNPROFOR (the acronym for the U.N. Protection Force) arrived in early 1992, they found themselves amid a full-scale civil war, with Croat, Muslim and Serb populations utterly at odds and fighting furiously. There was no peace to keep, and no way of enforcing one. U.N. forces were shot at, occasionally taken hostage and otherwise exploited for military or propagandistic advantage. Confronted in plain sight with the most egregious atrocities, U.N. forces stood by.

As I write this, the once-beautiful medieval city of Mostar, famed for its bridge over the river Neretva, comes to mind. On one side were Muslims, on the other Croats. The buildings on either embankment had been whittled away into eerie Giacometti-like sculptures by incessant weapons fire. Parks became cemeteries, with little room even for trees.

Lebanon could be more like Bosnia than Kosovo. Rather than welcome a U.N. force in the name of peace, Hizbullah will see them as stooges for Israel. They would be occupiers, enemies to be warily tolerated at best and more likely to be fought. Any “buffer zone” created by the U.N. would be honored in the breach. If it’s the relatively narrow 1.2-mile-wide zone Israel says it will create, then the international force would be left hunkered down in a sort of no-man’s land. The predictable result: incidents such as yesterday’s killing of four U.N. observers by Israeli artillery. A larger zone—say the area south of the Litani River—would be difficult if not impossible to police. At the very least, Hizbullah will go underground and commingle with Lebanon’s civilian population, now and again firing off longer-range missiles toward Israel, just to prove they are still there, unbowed and unconquered. Israeli demands that the U.N. confront the perpetrators will set the stage for violence.

Because only Iran and Syria can control the militias, a U.N. force would be at their mercy. What price will Tehran and Damascus extract for keeping Hizbullah quiescent? It’s probably no coincidence that intimations that Iran might be willing to mediate comes as the Security Council takes up the matter of Tehran’s nukes. Washington wants to send a clear message to the mullahs: stop your nuclear program now, or else. But will a Europe that wants a ceasefire in Lebanon (and whose troops are on the ground) be willing to go along? Inconceivable as it might sound, a nuclear Iran could be the by-product of a ceasefire in Lebanon.

Syria, too, will exact its price, no doubt seeking to restore the influence it lost in Lebanon when its intelligence and army forces were withdrawn following last year’s all-too-brief Cedar Revolution. Making her diplomatic rounds in Beirut and Jerusalem, Rice spoke about the inevitable pain of a “new Middle East” being born. No doubt she believes the child of this bloodshed will be named democracy. But others deem that far-fetched. From Amman to Riyadh come warnings of a looming regional war and a sharp radicalization of the region’s politics. A new Middle East may be aborning, but we should fear what rough beast it will turn out to be. This much is sure: an international peace force will have trouble restraining it.