WASHINGTON, 7 April 2003 — The new ability of US military forces to move at will in Baghdad — demonstrated especially by Saturday’s startling “thunder run’’ of 70-ton army tanks through the heart of the city — suggests that a major “tipping point’’ has been reached: US troops no longer are fighting a conventional enemy and are waging a less intense but still dangerous “checkpoint war.’’

This isn’t to say that the war is over, or that the remaining fighting will be quick or easy. Diehard fighters in the capital almost certainly will be able to mount some ambushes on US troops. The Americans also face daunting challenges tracking down President Saddam Hussein and his close circle of advisers.

But the battle is entering a distinct new phase. It now appears unlikely that there will be many more tank battles or area bombings, the classic signs of all-out war. Signs indicate that large-scale organized resistance is waning, with Saddam Hussein’s military no longer capable of coordinating a large-scale counteroffensive, and the bulk of its forces killed, deserted, or in hiding.

“There’s not much sign of the regime,’’ said one US intelligence official, commenting on the remains of the Baath Party and Saddam’s inner circle.

The commander of the US air campaign in Iraq indicated Saturday that the conventional military threat outside Baghdad is pretty much finished. “Our sensors show the preponderance of the Republican Guard divisions that were outside of Baghdad are now dead,’’ Air Force Lt. Gen. Michael “Buzz’’ Moseley said in a telephone news conference. He added: ‘’ I find it interesting when people say we’re softening them up. We’re not softening them up. We’re killing them.’’

The war that remains, whether it lasts a week or a year, is in some ways more complex, ambiguous and difficult. Drawn-out operations with murky results, as in recent months in Afghanistan, tend to lose the attention of the US public and the representatives they send to Congress. Also, when such ambiguous operations produce a steady trickle of casualties, it tends to erode public support. Those tendencies especially worry the army, which fears that it will be left responsible for peacekeeping when the other services declare their wars over and head home.

In the short term, a major worry is the whereabouts of Republican Guard units that seem almost to have melted away into Baghdad. “There certainly are some remnants of the Republican Guard’’ in the capital, Air Force Maj. Gen. Victor E. Renuart Jr., operations director for the US military headquarters in the war, said at a briefing in Qatar.

An additional concern is the foreign fighters — apparently Egyptians and Jordanians — reported by Marines moving toward the capital from the southeast to be involved in the fighting. If there are more of those would-be martyrs waiting in the capital, the fighting there could involve many more suicide attacks.

Also, the lean size of the US invasion force is likely to restrain the scale of operations, said retired Army Col. Robert Killebrew, an expert in military planning.

The army’s 4th Division has been deployed to the region to help, but Killebrew added: “More troops will be required eventually even then.’’

Finding those troops for the rest of the war and then for the post-war peacekeeping is likely to strain the army, which calculates that Iraq duty will keep at least two divisions busy for several months. Beyond that, another division will be training to replace them. All told, that is a considerable demand on a service that has just 10 active-duty divisions and already has troops busy in Afghanistan.

Nor is post-war duty alluring for the rank and file. The military adage is that peacekeeping isn’t a soldier’s job but only a soldier can do it. For troops trained for combat assaults, it can be demoralizing to stand endless duty at checkpoints, bored for months and then momentarily terrified when it appears that a car bombing is imminent. Soldiers on semi-occupation duty in a violent environment are frequently torn between firing in self-defense and the knowledge that they are firing on civilians. A split-second decision made by a corporal at a checkpoint can have international repercussions — especially if the soldier guesses wrong, and there is no bomb in the car.

In some ways, that isn’t a sharp departure. The vast majority of US military deaths in the war so far haven’t been in traditional force-on-force high-intensity combat, but in smaller actions, such as car bombings, ambushes and in sniping on convoys. Those sort of attacks mainly have been in rear areas, along supply lines. But now, with no “front’’ remaining, the entire country of Iraq is essentially a rear area.