When Turkey's reluctant parliament ruled out a true northern front for the war in Iraq, Pentagon war planners turned to a small, lightly armed U.S. force for the largely Kurdish north, hoping the bluff could pin down Iraqi troops and keep the hostile Turks away from the Kurds while the real fighting force pushed toward Baghdad from Kuwait.

But even that modest plan has been overtaken by the rapid collapse of Saddam Hussein's regime. Now, defense officials say, war planners are more or less "winging it."

Two battalions of the 173rd Airborne Division hustled out of positions inside the Kurdish autonomous zone and into the strategic city of Kirkuk yesterday, after Kurdish pesh merga fighters and a few U.S. Special Operations soldiers rolled into the city virtually unopposed, said Maj. Gen. Stanley A. McChrystal, vice director for operations for the Joint Chiefs of Staff. That wasn't according to plan, a defense official said afterward -- but in the last few days, not much has been.

"Any time things start falling apart fast, you wing it," the official said. "This is all going faster than the plan."

War planners originally hoped to squeeze Baghdad in a vise, with the 4th Infantry Division rolling south from bases in Turkey and the 3rd Infantry Division and Marines pushing north from Kuwait. After Turkey blocked any troops from attacking from Turkish soil, Pentagon officials repeatedly boasted that there would be a northern front.

But that was more bluff than threat, defense officials say. The idea was to field as many as 2,000 Special Operations forces to bolster Kurdish fighters and call in airstrikes on Iraqi positions. The 173rd Airborne would make a grand entrance from Italy, parachuting onto a friendly airfield in the Kurdish autonomous zone. Those 2,000 troops would be bolstered by a small armored contingent. That all happened, and this week, about 300 soldiers from the 1st Infantry Division arrived with six M1-A1 Abrams tanks, six M2 Bradley Fighting Vehicles and an assortment of Humvees and trucks aboard 20 to 25 C-17s, a defense official said.

But even that reinforced deployment was never meant to seriously challenge the Iraqi armored divisions defending Kirkuk, Hussein's power base in Tikrit or the northern approaches to Baghdad, defense officials say. Instead, they were supposed to present just enough of a fighting force to keep Iraqi troops in the north from falling back to reinforce Baghdad, while standing between the independence-minded Kurds and a Turkish army determined to thwart an independent Kurdistan.

Even a small force could dissuade Turkey from attacking, officials believed, if Americans could potentially be caught in the crossfire. "The politics of having U.S. forces caught in the middle of those two forces would have been too much," a Pentagon official said.

To a large extent, it worked, Pentagon officials said. The 11 regular Iraqi army divisions that once lined the so-called green line separating Hussein's Iraq from the Kurdish-controlled north, stayed put, even as they were decimated by airstrikes directed by Special Operations forces. Those 11 divisions have been reduced to "elements of eight," a defense official said. The combat readiness of that force -- which was questionable even before the war -- has been degraded significantly, McChrystal said.

The two northern Republican Guard divisions did retreat to Baghdad to reinforce Republican Guard troops defending the capital from the south, but that reinforcement proved ineffectual against U.S. air power and artillery.

And so far, the Turkish army has stayed out of the Kurdish-controlled area.

With Iraq now descending into anarchy and the war effort moving toward Hussein's home town of Tikrit, defense officials say they are having to become more flexible with their northern Iraq war plan. Special Forces units yesterday moved on the city of Mosul and have sealed off routes in and out of Tikrit. The 173rd Airborne's move into Kirkuk yesterday was aimed at reassuring Turkey that the United States does have some semblance of control and will not allow Kurdish forces to make the city their de facto capital.

But a senior defense official made it clear that U.S. forces in the north would not be leading the charge on Tikrit in the coming days, at least not as long as pockets of resistance in Baghdad continue to exact U.S. casualties in the capital.

"We'll let the north work itself out a little, pick at it as we can," the official said. "But there's still a lot of fighting to be done in Baghdad."

When the final push on Tikrit comes, it is likely to be spearheaded by the 4th Infantry Division, which has finally arrived in force in Kuwait after its equipment was diverted from Turkey, defense officials said. One official said a brigade-sized task force of around 5,000 would be ready to roll north in two days. A final decision on whether that task force reinforces troops in Baghdad or presses on to Tikrit may not be made until it reaches the capital. But one defense official said the 4th Infantry Division vanguard likely would blast past Baghdad toward Tikrit if resistance continues to dissipate. The 101st Airborne Division, now reinforced by a 500-man task force from the 2nd Armored Cavalry Regiment, faces the same options.

Meantime, McChrystal said, U.S. air power will continue to pound command-and-control facilities and the remnants of Hussein loyalists in Tikrit. Air Force Secretary James G. Roche told CNN yesterday, "We've done a lot in Tikrit, but there's no reason to reduce every building to rubble."

By the time U.S. forces reach Tikrit, there may be little left to fight, one defense official said, concluding, "It's going to be 'Shock and Awe, Part Deux.' "

A soldier of the 173rd Airborne Brigade patrols an area near an airstrip in Harir. Two battalions of the 173rd moved into Kirkuk yesterday.