WASHINGTON, March 17 — In January, when President Bush announced his plans to reinforce American troops in Baghdad, Shiite militias were seen as the main worry. Some analysts predicted that bloody clashes with Shiite militants in the Sadr City district in northeastern Baghdad were all but inevitable.

Instead, during the early weeks of the operation, deadly bombings by Sunni Arab militants have emerged as a greater danger. In particular, the threat posed by the Sunni group Al Qaeda in Mesopotamia was underscored when American troops seized a laptop computer from a senior operative in the group who was killed in late December.

Information from captured materials indicates that the group’s leadership sees “the sectarian war for Baghdad as the necessary main focus of its operations,” according to an intelligence report that was described by American officials.

Reflecting concern over the bomb attacks, especially car bombings, American military officials have begun to emphasize that bringing security to the Iraqi capital will involve not only the protection of Baghdad neighborhoods, but also raids to shut down bomb factories and uncover arms caches in the largely Sunni areas on the outskirts of the city.

“The Baghdad belts are increasingly seen as the key to security in Baghdad,” Lt. Gen. Raymond T. Odierno, the American officer in charge of day-to-day operations in Iraq, said in an e-mail message. “I believe this is where you can stop the accelerants to Baghdad violence. We have already found a large number of significant caches in these areas related to car bombs and I.E.D.’s,” or improvised explosive devices, commonly known as roadside bombs.

“The Shia have gone to ground for the most part, but there are still rogue elements of Shia extremists that are still a threat and conducting operations against the coalition, but more importantly against the government of Iraq,” he added.

The threat has shifted on the eve of the fourth anniversary of the start of the war in Iraq, in which American forces toppled Saddam Hussein only to face a growing insurgency and find themselves involved in an arduous effort to head off growing sectarian strife.

In its efforts to stabilize Iraq, American commanders have had to contend with Al Qaeda in Mesopotamia, other Sunni Arab insurgent groups, a variety of Shiite militias, criminals and, they say, Iranian operatives. The greater Baghdad area seems to include all of them, making the mission there one of constant adjustment to adversaries who are revising their own tactics.

According to American intelligence analysts, Al Qaeda in Mesopotamia’s Baghdad strategy has gone through several changes. The overwhelming majority of the group’s members are believed to be Iraqi. But some senior commanders are foreigners, including Abu Ayyub al-Masri, an Egyptian who became the leader of the organization last year after the death of Abu Musab al-Zarqawi, the Jordanian terrorist who founded the organization.

The group has been active in the Sunni-dominated Anbar Province in western Iraq. But it has also long operated in the Sunni areas on the outskirts of the capital. Mr. Hussein encouraged the settlement of Sunnis in these areas in the hope that it would protect his government, and some towns and rural communities there have emerged as havens for Sunni militants.

In the summer and fall of 2006, the group’s leaders saw an opportunity to step up the fight in Baghdad against Shiite militias, American troops and the nascent Iraqi security forces, according to captured documents. Some of the insight into the group’s strategy was obtained from the laptop computer seized when a senior Iraqi adviser to Mr. Masri was killed by troops of the American-led forces in late December at a traffic checkpoint.

The adviser, who among other aliases used the name Abu Hasan, was detained by the multinational troops in January 2005 but inadvertently released because his role in Al Qaeda in Mesopotamia was not well understood at the time.

As outlined in the captured documents and other material that was seized, the group’s initial strategy was to push Shiites out of western Baghdad. As part of the sectarian battle for the capital, the strategy also called for attacking Shiites in parts of nearby provinces, specifically southern Salahuddin, western Diyala and eastern Anbar, attacks that the group’s leaders also calculated would put American and Iraqi troops on the defensive. (The documents, American officials say, also reflected a continued interest in obtaining chemical weapons.)

But Shiite militias, particularly Mahdi Army operatives, responded with their own offensive, forcing the Sunni militants to retreat. A Pentagon report to Congress noted in November that the main Shiite militia group, the Mahdi Army, had replaced Al Qaeda in Mesopotamia “as the most dangerous accelerant of potentially self-sustaining sectarian violence in Iraq.” American forces, instead of withdrawing from the capital as the Sunni insurgents had hoped, prepared plans to reinforce their troops there.

According to captured memos portrayed in American intelligence reports, the group was frustrated with the Shiite militias’ success, was unhappy with weapons shortages and was somewhat disorganized, according to an account by an American official who asked not to be identified because he was discussing intelligence matters.

As a result, the organization adjusted its tactics. It began to rely more on the Sunni enclaves on the outskirts of Baghdad. Senior leaders rotated through the areas in order to direct operations while lower-level fighters operated in the capital.

Car-bomb components are also made in the surrounding Sunni areas and then smuggled into Baghdad, where they are assembled in the hope of killing Shiites and escalating sectarian violence. In a reflection of the group’s tactical shift, car bombings have greatly increased this year, reaching a peak in Iraq in January and February, according to Maj. Gen. William B. Caldwell IV, the top spokesman for the United States military. Of the 77 car bombings in Iraq in February, 44 were in Baghdad.

In addition to car bomb attacks, the group’s basic tactics are to attack American and Shiite militia supply lines. When faced with American combat power, the Sunni militants tend to disperse, hoping to fight another day, American commanders say.

Col. J. B. Burton, the commander of the Second Brigade Combat Team for the First Infantry Division, told reporters on Friday that the Sunni militants were also taking advantage of the decision of some Shiite militias to become less active or leave Baghdad. Sunni militants “have seen an opportunity with Shia extremists out of the area to strike with much violence,” Colonel Burton said. “What we have seen is when the Shia extremists departed our area of responsibility, specifically in western Baghdad, incident rates in the Shia areas dropped dramatically,” he said. “Incident rates in the Sunni areas increased a bit with vehicle-borne improvised explosive devices targeting Shia gathering places and Iraqi security force locations.”

American military officials said they had long understood that Baghdad could not be secured without also controlling the surrounding suburbs, but the car bombings have made this all the more important. Among other areas, Sunni enclaves are located in the belt from Yusufiya to Salman Pak south of the capital and in towns like Baquba in Diyala Province north of the city. An American battalion was recently sent to Diyala as a result of sectarian fighting and Al Qaeda in Mesopotamia operations there.

Gen. David H. Petraeus, the senior American commander in Iraq, alluded to the problem in his news conference last week, noting that two car-bomb factories had recently been discovered in Salman Pak, to the southeast, and in Karma, northwest of Baghdad.

“They tend to be in the outskirts in these very rural areas, small villages and outlying houses and farms, and so forth,” General Petraeus said. “And we clearly have got to find as many of those as we can to destroy them and then, obviously, to interdict those that are still able to be built.”

“Although the focus, the priority, clearly is Baghdad, anyone who knows about securing Baghdad knows that you must also secure the Baghdad belts, in other words, the areas that surround Baghdad,” he said.