"On a Bridge to Unity, Or Perhaps to War" "AIMMA BRIDGE, the Bridge of the Imams, is an unremarkable concrete and steel span over the muddy waters of the Tigris. But in recent days, a singular tragedy has made it a symbol of the contest between two contradictory impulses competing to determine Iraq's future: the tensions between Iraq's two main religious groups that could split the country apart, and the common national culture that, in spite of everything, still has some power to bind Iraqis together. The bridge links Kadhimiya, a Shiite Muslim neighborhood named for its shrine to an eighth-century Shiite saint, and Adhamiya, a Sunni Arab area named for a Sunni imam, which has been a stronghold for insurgents. On Wednesday, a stampede broke out on the bridge in a procession of Shiite pilgrims, leaving close to 1,000 dead. Though the immediate cause was panic over unfounded rumors that a suicide bomber was on the bridge, many Shiites instantly began blaming Sunni insurgents, who had planted fear in the crowd by attacking the shrine in Kadhimiya an hour earlier. Immediately, many Iraqis feared a furious Shiite reaction. But just as quickly the disaster seemed to revive -- if only tenuously -- a language of Iraqi unity. As pilgrims tumbled off the bridge, scores of young Sunni men from Adhamiya leapt into the river to save them. Sunni leaders who had assailed their Shiite and Kurdish political counterparts only a day before called on residents of Adhamiya to do all they could for the wounded. And for a day or two, insurgent violence seemed to subside. Some Iraqis used the disaster as a reminder that for all the divisive politics of recent weeks, most Iraqis still do not define themselves first and foremost by sect. ''I think what happened brought people together, just because it was such a colossal tragedy,'' said Adnan Pachachi, a secular Sunni who was Iraq's foreign minister before the rise of Saddam Hussein. ''You must remember that until recently most Iraqis didn't speak this sectarian language.'' That is true, but it is also true that Sunni-Shiite antagonisms, suspicions and jealousies are real, and have deep historical roots. After the stampede, for example, word spread among Shiites that many pilgrims had collapsed after being handed poisoned food and drinks by Sunnis as they passed through Adhamiya toward the shrine. It was impossible to determine whether such poisonings took place -- Iraq's health minister said he could not confirm them -- but in the minds of many Shiites that almost didn't matter. The rumor played right into their suspicions. Shiites believe that Sunni agents poisoned Imam Musa Kadhim, the saint whose death they were commemorating. And for many religious Shiites, their suffering at the hands of Sunnis is all one, whether it happened 1,200 years ago or yesterday. Still, cries of revenge were sparse, even when, on Thursday, gunmen in Adhamiya fired on Shiite mourners placing candles at the bridge. Iraqi officers fired back, and no mourners appeared to have been hurt. On Friday, gunmen fired on two Sunni mosques in southern Iraq. For the most part, Shiite religious leaders counseled forbearance and the Shiite faithful obeyed. But events of recent months hint that Shiite forbearance may be at its limit. Sunni attacks have become increasingly common, and even before the bridge deaths, some Shiites had begun striking back. Scores of Sunnis have been abducted across Iraq, only to be found dead later. Many Sunnis blame militias controlled by Shiite religious parties. After a particularly bloody bombing in July, even Grand Ayatollah Ali al-Sistani, Iraq's most senior Shiite religious figure, seemed to step outside his usual counsel of caution, urging the government to ''defend this country against the mass annihilation.'' So it came as a surprise to some when grief over the stampede seemed to impose some unity on the country, with leaders of all parties expressing their condolences. By Friday prayers, however, when imams across Iraq began explaining the event to their communities, differences began to reassert themselves. At the Shiite Baratha mosque in Baghdad, Jalal Adin al-Sageer blamed insurgents, saying they had poisoned the pilgrims as well as firing on the shrine. He seemed to suggest an equivalence between the perpetrators of the disaster and the Sunni Arab representatives who refused to grant approval to Iraq's new constitution. And he criticized the government for not controlling and guarding the crowd better, singling out the Sunni defense minister, Sadoun Dulaimi. ''Some terrorists have penetrated the ministry of defense,'' the imam said. At the mosque formerly known as the Mother of All Battles Mosque, Abdul Ghafour al-Samarrae, a moderate Sunni cleric, angrily dismissed reports of poisoning as rumors spread by people who wanted to ''pour oil on the fires of sectarianism.'' He praised the young Sunni men of Adhamiya for heroic efforts to save pilgrims' lives. But then he too lapsed into the language of division, blaming the leader of the Interior Ministry, a Shiite, for the murder of 36 Sunnis last week. Yet even as they launched these attacks, the two imams spoke of sectarianism as the worst sin of all. Like many ordinary Iraqis -- even those with fierce sectarian agendas -- they still clung to what is a public article of faith here: that their country is a single, unified people. What was not yet clear was whether that language of national unity will prove strong enough to be a bridge linking Iraqis of different sects, or is already a dead end. THE WORLD"