BAGHDAD — The killers were on the way, and Ahmed Hassan had only a few hours to save his family.

On this day, August 6, 2006, at least 22 Iraqis would die in rising sectarian violence. Mr. Hassan, his wife, and their children would survive, but at a cost: They would lose their home, and flee their neighborhood.

Nineteen months later, they remain exiled in their own country. “I do not want to return to my house for the time being because I already lost my house and I do not want to lose my life,” Mr. Hassan said, his infant twins in his arms. More than 4 million Iraqi lives are in similar straits — upended by five years of war that has turned neighborhoods into killing fields and sent countless refugee convoys scurrying for the border.

About 2 million people have fled to other countries since the 2003 American-led invasion, mostly to Arab neighbors such as Jordan and Syria, according to the U.N. high commissioner for refugees. Another 2.4 million people have been displaced but are still in Iraq, some forced out during Saddam Hussein’s rule and others since the start of the war in March 2003.

A 2007 year-end report by the International Organization for Migration found that most internally displaced Iraqis left home because of direct threats to their lives.

The displacement crisis is one of the gravest consequences of the war and is among its festering legacies. Regardless of what happens on the battlefield in years to come, the sheer size of the uprooted population is already bigger than the dislocation after Hurricane Katrina struck New Orleans in 2005. Despite improved security here during the last half-year, attempts to allow people to return are in the earliest stages. Simple homelessness doesn’t fully describe the problem.

“It affects every aspect of someone’s life,” Karim Khalil, who analyzes the Iraq situation for the Internal Displacement Monitoring Centre in Geneva, said. “Access to food, and documentation, access to education, access to health and legal services. There are a whole panoply of issues.”

Mr. Hassan is part of Iraq’s Shiite Muslim majority, but members of all of Iraq’s ethnic groups have been thrust from their homes. This story could just as easily have been about 27-year-old Mohammed Abdul-Wahab, a Sunni auto mechanic who, like Mr. Hassan, is from the Jihad section of western Baghdad; he endured five months of joblessness after Shiite militants warned him and his family to leave their home in 48 hours, or face death.

Mr. Hassan, his wife, and their four young children lived in two-story home they’d inherited from Mr. Hassan’s father. Shiites and Sunni Muslims lived side-by-side in the middle-class area until waves of sectarian violence spread across the country after the bombing of a Shiite shrine in February 2006.

Around that time, Mr. Hassan joined in a procession to commemorate the seventh-century martyrdom of Imam Hussein, a revered Shiite figure. It was enough to earn him a death sentence.

Sunni neighbors tipped him on August 6 that insurgents were on the way to his house to kill Mr. Hassan for joining in the religious ritual.

One of those friends volunteered to hide the family. Then, in the afternoon, gunmen arrived outside their house. When they couldn’t find Mr. Hassan they became enraged, and burned the building down.

The family savings, all in cash, were in the house. Gone. The family’s possessions, too: clothes, furniture, everything but what they took into hiding.

Working in the Culture Ministry’s printing house, Mr. Hassan earned a modest living but nowhere near enough to replace what was lost.

Where to go? What to do?