The battle now beginning between U.S. forces and the Republican Guard's Medina Division to the southwest of Baghdad promises to be a decisive engagement that signals whether the new Gulf War will be over in a week or two or drag on for a month or more.

Until now, U.S. forces have fought regular Iraqi units and militias in small-unit skirmishes. Now they will face the Iraqi army's best troops for the first time, not in the wide-open desert but in the heavily populated and vegetated Euphrates Valley. Perhaps 40,000 troops and aircrews all told from both sides are poised to clash just a few miles west of the ancient city of Babylon. If Iraq chooses to use chemical weapons during this war, analysts think it will be in this battle.

"This engagement will determine if this is a long or short war," an Army officer at the Pentagon predicted.

The impending battle confronts U.S. forces with a dilemma that goes to the heart of the complex mission in which they are engaged: They can maximize the advantages of their overwhelming firepower and bomb a wily adversary hiding heavy weapons in built-up areas, which would inflict civilian casualties and set back the U.S. campaign for public opinion. Or they can try to attack precisely with low-flying helicopters and ground forces, which could mean losing more U.S. troops.

If the fight against the Medina Division ends in just a day or two, or if parts of the unit even surrender without a fight, that will send a powerful signal that the climactic battle for Baghdad won't be as difficult as some have predicted, or won't occur at all.

But if the 10,000-man Medina division manages to undercut U.S. momentum, and especially if it inflicts heavy casualties in the process, or if it just retreats from a battlefield strewn with dead civilians, then the tone of the war probably will change. A bitter fight that takes a week might even persuade the U.S. military to alter its strategy and dig in to wait for reinforcements from the Army's tank-heavy 4th Infantry Division -- which probably would take at least two or three weeks.

Early indications are that it will be a tough battle. In the first engagement between the U.S. Army and the Medina Division, before dawn yesterday, about 35 Apache attack helicopters flew over part of the division, which is spread out in wooded and built-up areas east of the town of Karbala, about 50 miles southwest of Baghdad.

Officials portrayed the foray as "reconnaissance by fire" and declared it a success. At a briefing in Qatar, Army Brig. Gen. Vincent K. Brooks said the helicopters "were very effective in their mission." But returning pilots sounded less certain, saying they hit a handful of tanks and armored vehicles but were forced to cut short their effort because of heavy ground fire. They also said their rules of engagement had prevented them from firing on some targets. One helicopter was downed either by fire or a mechanical failure, and its two crew members were declared missing.

Secretary of State Colin L. Powell, himself a retired Army general, sounded optimistic yesterday as he described in an interview with Fox News how the U.S. offensive would defeat the Medina Division. "After the ground forces have fixed them, air power goes after them, and then the ground forces go in and finish them off," he said.

That's the Army's preferred mode of combat, used to devastating effect in the first Persian Gulf War, during which Powell served as chairman of the Joint Chiefs of Staff. This time, however, the engagement will take place in vastly different terrain from the open desert where that war was fought, and could involve the weapons greatly feared then, but never used.

"There are intelligence reports that Iraq has distributed chemical weapons, most likely VX [nerve gas] to the Republican Guard," including the Medina division, said Kenneth M. Pollack, a former CIA analyst of Middle Eastern militaries. When reconnaissance images showed munitions being delivered, he said, they were accompanied by chemical decontamination trucks.

Retired Rear Adm. John Sigler, a former chief planner for the U.S. Central Command, agreed with that assessment, saying, "I don't think you'll see bugs [biological weapons], but you might see gas."

Indications are that Iraqis now are applying hard lessons learned during the Gulf War and then, by the Yugoslav military, during the 1999 Kosovo air campaign. The tanks and heavy weapons of the Medina Division weren't arrayed for battle, in tight formations that would make them easy targets, but instead were dispersed under trees and in the farming villages of the Euphrates River valley, defense officials said.

That setup makes them difficult to hit with punishing B-52 carpet bombings. Instead, Air Force A-10s and F-16s and Navy F/A-18s are flying smaller "tank-plinking" strikes, which are riskier for pilots. While those retail-style raids do destroy some armored vehicles, they don't have the effect that heavy bombers do of disrupting other essentials of military operations, such as resupply and communications.

All told, half the sorties flown in the past two days were aimed at Iraqi military units, mainly the Republican Guard, the official said.

Smaller than a U.S. armored division, which typically would have 15,000 to 20,000 troops, the Medina Division has about 10,000 troops manning 250 tanks, about 250 armored personnel carriers and perhaps 60 artillery pieces, Pollack said. Some of that artillery, he noted, is the same type as that being fielded against it by the 3rd Infantry Division -- tank-like U.S.-made M109 self-propelled howitzers captured during the Iran-Iraq war. Also, the Guards, unlike regular Iraqi units, are equipped with SA-14 and SA-16 handheld surface-to-air missiles, the Russian version of U.S.-made Stinger missiles.

The unit has a reputation of being disciplined and well-led dating to the Iran-Iraq war, when it spearheaded five complex offensives that, according to Pollack's 2002 study of "Arabs at War," forced Tehran to accept a ceasefire that ended the eight-year struggle.

In 1990, the unit was one of the four Republican Guard divisions that invaded Kuwait. It executed a flanking maneuver, entering the emirate from the west with the intention of surprising any Kuwaiti defenders on the side.

The division had a harder time seven months later during the Gulf War. A brigade of the U.S. Army's 1st Armored Division ran into it Feb. 27, 1991, near the end of hostilities, and destroyed about 70 T-72 and T-55 tanks and scores of other vehicles. The U.S. forces lost one soldier in the process.

Staff researcher Robert Thomason contributed to this report.

Members of the 15th Marine Expeditionary Unit set up a position during a scouting mission at a village in Az Zubayr in southern Iraq. U.S. forces are now facing the Iraqi army's best troops for the first time.