WASHINGTON, 26 March 2003 — As US armored columns drive toward Baghdad with the single-mindedness of a harpoon, deliberately bypassing Basra and other cities in southern Iraq, they are taking a calculated risk.

The downside of the strategy is apparent: Iraqi soldiers and paramilitary fighters are finding it easier to attack allied forces bringing up the rear and their thinly protected supply trains. But the Pentagon maintains that it’s all part of the plan.

Like Gen. Douglas MacArthur “island hopping’’ in the South Pacific during World War II, allied commanders chose to open the invasion of Iraq by sidestepping potentially bloody, time-consuming battles to occupy Basra, Nassiriyah and other population centers in southern Iraq — areas they considered strategically peripheral.

Instead, they focused on their central goal: Getting their most powerful forces — hundreds of Abrams M1 tanks, armored fighting vehicles, artillery and attack helicopters — to the gates of Baghdad in record time.

The gamble was that enemy forces would not be able to mount significant attacks along the allies’ stretched-out supply lines or otherwise divert attention from the campaign to the north. Allied forces rolling in behind the heavy armor that led the assault would mop up what initially seemed to be scant resistance. But recent developments have cast a pall over the early successes.

The abrupt surge in bad news has given rise to questions:

• Has the American battle plan come unraveled?

• Was it misconceived from the start?

• If Saddam Hussein has forces in southern Iraq capable of bloodying allied units, why were they not engaged and destroyed at the outset?

Monday morning, when Gen. Tommy Franks, chief of the US Central Command, briefed reporters at his headquarters in Doha, Qatar, a reporter from Abu Dhabi confronted him: “In the beginning of the war, so-called coalition forces claimed taking full control of Umm Qasr, then Nassiriyah, and yesterday Basra; and apparently, it seems now, it’s not correct,’’ the reporter declared. “Are you practicing a strategy of lies and deception, or have you just have been trapped by Iraqi Army?’’

The answer, Franks and many outside military analysts insist, is that recent resistance was expected, will be dealt with in coming days, and will be a small price to pay for the benefits of their determined thrust toward Baghdad. Recent problems have loomed large only because the early days made the war look too easy, military officials say.

“As you know, our forces have been moving rapidly. We have intentionally bypassed enemy formations to include paramilitary and the Fedayeen,’’ Franks said.

“We know that the Fedayeen has in fact put himself in a position to mill about, to create difficulties in rear areas, and I can assure you that contact with those forces is not unexpected,’’ Franks said.

Even so, some observers suggest that allied planners are making a mistake by not focusing more on securing key sites in southern Iraq. They point out that it increases the vulnerability of the 300 mile-long supply line on which the two forward units — the 1st Marine Division and the 3rd Infantry Division — must now rely.

What’s more, stiff Iraqi opposition in the port town of Umm Qasr has blocked the movement of critical food, water and other humanitarian aid to an increasingly desperate civilian population. The longer it takes to begin this distribution, some say, the greater the dangers of political instability. Even as Iraqis in the south continue to wave at US and British troops, a growing number are seething with anger as they struggle for basic necessities. In Basra, the country’s second-largest city, water has been cut off.

“I’m looking to see what happens in the south,’’ said Andrew Brooks, an analyst at the International Institute of Strategic Studies in London. “If they can’t control Basra easily, then what does that say about Baghdad?”

But the strategy for the war, including the decision to bypass population centers in the south, reflect a set of military and political conclusions reached by US planners months before the invasion began:

First, allied forces had to deal such heavy blows to Iraqi forces in the opening days of the invasion that regular army units and perhaps even some of the more disciplined Republican Guards would realize the futility of resistance.

Blitzing through to the outskirts of Baghdad with US heavy armor was considered a key element in achieving this goal. It would, in effect, put the point of the allied spear directly at the throat of the regime’s defenders.

Baghdad is “the Iraq center of gravity,’’ one military officer said soon after the invasion began. There is little sense in striking peripheral targets, he said, referring to the cities in the south.