If Iraq slips toward civil war, this town along the Sunni-Shiite fault line will be one of the flash points. Talking to U.S. troops at a base near here, you come away with a idea of what the war looks like out in the killing zone -- and how hard it is to mesh U.S. strategy with the nightmarish reality of the Iraqi insurgency.

This war is in many ways a series of disconnects, and you sense them during a visit to Forward Operating Base Kalsu, as the Army calls its garrison here. It's a war in which U.S. troops remain upbeat, even as support deteriorates back home; in which the appearance of stability in much of Iraq is shattered by spasms of hideous violence; in which U.S. military strategy is confounded by Iraq's political disarray.

Flying into Kalsu Base, the Black Hawk helicopter skims over the southern part of Anbar province, which is home to the Sunni insurgency. You imagine this as a landscape of ruin and insurrection, but it's actually a rich farming area with neatly arrayed fields and irrigation systems and crops ripening for the fall harvest. Needless to say, that tranquil impression hides the reality of the insurgency nested in these farmhouses and green fields.

Kalsu Base was set up back in May 2003 to protect the main convoy route to Baghdad, which was already beginning to come under attack from the fledgling insurgency. (It's named after Lt. Bob Kalsu, a professional football player who died in the Vietnam War.) Now it's home to a Mississippi National Guard unit, the 155th Brigade Combat Team, known as the "Mississippi Rifles." Their area of responsibility stretches south to the Shiite holy cities of Karbala and Najaf and north to the Sunni insurgent strongholds of Anbar province.

It's a spartan camp, far from the relative luxury of the Green Zone. The troops salute visitors with a rousing shout of "Dixie," although the unit's commander today is an African American, Brig. Gen. Augustus L. Collins. About 80 of the troops were sent home on emergency leave after Hurricane Katrina hammered the Mississippi coast. It's impossible to know in a brief visit whether more soldiers feel they should have been back home, but I didn't hear any grumbling.

Straddling the Sunni-Shiite fault line, the troops here see the two realities of Iraq. The Shiite areas to the south are fairly calm; Iraqi military and police units, nearly all Shiite, are increasingly effective in keeping the peace there. Najaf, for example, is protected by six checkpoints manned by Iraqi police. Lt. Col. James Oliver, who has responsibility for these areas, says that he hopes to be able to turn over Karbala and Najaf provinces entirely to Iraqi control by the end of October.

The Sunni areas to the north are ground zero for the insurgency. The fighters travel easily from place to place, sheltered by an intimidated local population. Tribal and clan chieftains operate as a kind of local mafia, selling their services to the well-financed insurgents. There is no effective Iraqi army or police presence in these Sunni areas. Nor is there a Sunni militia that might maintain a rough peace, the way Shiite and Kurdish militias have done in their areas.

The commanders here say their strategy is to form what they call an "arc of stability" across the Sunni-Shiite divide and then gradually push it north. It sounds like the "oil-spot strategy" for Iraqi counterinsurgency that is outlined in the current issue of Foreign Affairs by analyst Andrew F. Krepinevich Jr. The problem is that the oil spot here is almost entirely Shiite, and to expand it into Sunni areas might produce pitched sectarian warfare. Adding more American troops wouldn't help much.

To spark hatred along the fault line, the insurgents stage brutal attacks on civilians, including one the day I visited. Near Kalsu Base, in a village called Muelha outside Iskandariyah, terrorists broke into an elementary school. They seized five schoolteachers and a bus driver, all Shiites, took them into a room and shot them dead. The Mississippi Rifles were just a few miles away, but light-years distant in terms of their ability to stop the appalling violence.

The soldiers at Kalsu Base are buying time. As long as they guard the fault line, a full-scale civil war is unlikely in this region, says Lt. Jennifer Bowen, the deputy intelligence chief. But if they leave, a sectarian conflict is likely. Before the Mississippi Rifles go back home at year-end, the Iraqis will hold another election. If the new government doesn't reflect a Sunni-Shiite alliance that can begin to restore order, sending a new team of Americans to Kalsu Base won't make much sense.