Having never grown up in a civil-war environment, I don't really understand the actual dynamics involved, how things progress from peaceful co-existence to neighbors gunning down neighbors in the street. Take Iraq. For years Sunnis and Shiites lived together peacefully—in urban areas especially, the two groups often intermarried, and sectarian identities often played little role in everyday life. There may be differences between the two groups, but there aren't vast reservoirs of inter-group hatred waiting to be unleashed. And even nowadays, as Zarqawi and his crew try to foment civil war with targeted killings, Shiite leaders—especially Ayatollah Sistani—are insisting again and again that Shiites must resist the impulse to retaliate against Sunnis. So a lot of very concrete obstacles lie between Iraq's current state and all out, inter-sectarian civil war.

And yet these things happen all the time—Rwanda, Yugoslavia, the list goes on. How does it happen, exactly? I imagine there are entire books written on the subject, but in the meantime, check out this recent Time article, which lays out the growing animosity between Sunnis and Shiites unfolding in Baghdad, bit by bit. It's a chilling story:

A murder spree has erupted in Washash, as in countless neighborhoods across Baghdad. Death squads, which tend to move in Opel sedans, are entering what once were tight-knit communities and killing ordinary citizens, apparently to stir up sectarian hatred. The goal: to incite a civil war that each side hopes will give its sect dominance over the other…. [T]he killings in Washash are a grim portent of the kind of chaos that may lie in Iraq's future, whether or not U.S. soldiers stay on in force. "If the U.S. troops leave, we will have a civil war," says a Sunni ex-army officer who prefers not to reveal his name. "It will go on until one sect wipes out the other." ....

Despite the brutality of Saddam Hussein's regime, Sunnis and Shi'ites in Iraq tended to live in relative harmony. Although the sectarian split occurred early in Islamic history and concerns a critical disagreement over who was the rightful successor to the Prophet Muhammad, members of the two groups often trace their roots to the same Arab tribes and frequently intermarry.... Sunnis and Shi'ites played on the same sports teams and shared hubble-bubble pipes over domino games in cafes. "These two words--Sunni and Shi'ite--didn't exist for us," says Walid Ahmad al-Anei, a Sunni. "We were all Muslims."

But these days, as Walid learned to his horror, the division is all too real. ... As more Sunni extremists poured in from abroad to join the insurgency, they tapped into latent anti-Shi'ite feelings among Iraq's Sunnis, prompting some to resort to violence. Abu Mousab al-Zarqawi, the terrorist who heads al-Qaeda's operations in Iraq, fanned the flames, denouncing Shi'ites as worse infidels than the Christian "crusaders," as he refers to the U.S. troops. Shi'ite groups like the Badr Corps, whose militias are apparently armed by Shi'ites in Iran, have responded with equal savagery against the Sunnis. ....

These days Sunni and Shi'ite friends still sometimes sit together in the cafes, but the carefree ways of the past are gone. "Beneath our smiles, our hearts have closed," says a former army officer, a Sunni. "We no longer trust them, nor do they trust us." Residents believe the killers come from outside Washash, but they know there are informers within. Armed Shi'ite vigilantes patrol the streets, questioning strangers.

The whole story gives a more detailed account, so take a look.