With American troops rolling through Baghdad, U.S. military leaders are asking a disturbing question: Have Iraq's Republican Guards really melted away, or are they regrouping to fight another day?

Early fears about the battle for Baghdad raised the prospect of house-to-house combat and even chemical warfare. But U.S. forces quickly overran the capital.

Upon entering the city, Marine Cpl. Nate Decavelle wondered out loud with a yawn: "Where are the Iraqis at?"

The Republican Guard, the country's best-trained and best-equipped military units, were mostly a no-show, abandoning tanks, guns and even uniforms even before the U.S. sweep.

But as the focus of the war shifts to the suspected stronghold of Iraqi loyalists in the northern city of Tikrit, President Saddam Hussein's hometown 100 miles north of Baghdad, some warn that it is too soon to write off Iraqi forces.

One U.S. official involved in both military operations and intelligence said there are thousands of Iraqi troops unaccounted for.

"That's the scary part. We don't know where these guys went to. Did they just melt into the population? Are they planning to come back out as paramilitary? Are they laying in wait?" the official said, speaking on condition of anonymity.

Lt. Gen. Wafiq al-Sammarai, former head of Iraqi military intelligence who defected in the '90s and is with the Iraqi opposition, said in an interview with the Arab TV network al-Jazeera on Thursday that he expects the Iraqis' northern front to fall within the next two days at most. He predicted opposition forces will link up with the Americans in Baghdad by then.

"It is illogical for the regional command to continue in the northern region, if the leadership in Baghdad was killed, liquidated, fled or disappeared," he said. "It is not supposed to continue to operate. If they do, this is madness."

Before the battle for Baghdad, U.S. military commanders estimated that U.S. airstrikes had wiped out about half the strength of the remaining four Republican Guard divisions. And Gen. Richard Myers, chairman of the Joint Chiefs of staff, said all but a couple dozen of the Iraqis' tanks had been destroyed in less than three weeks of combat.

A statement -- reportedly from Saddam himself -- seemed to verify the disarray, calling on troops separated from their units to join up with other units to fend off the Americans.

U.S. military officials privately acknowledged they overestimated their enemy. Iraq's military machine, once billed as the world's fourth-largest, lost half its might during the 1991 Gulf War. Its tanks were outmoded, and what was left of its air force did not dare to fly this time.

Brig. Gen. Vincent Brooks, deputy operations director at U.S. Central Command, said many Iraqi fighters simply decided not to die for a crumbling regime. But he also insisted the U.S. military cannot assume they decided to "just walk off the battlefield and never fight again."

In fact, Brooks said Wednesday that Iraqi reinforcements seemed to be converging on Tikrit.

Coalition aircraft are hitting hard at the Republic Guard's Adnan division there. And coalition roadblocks are also trying to prevent Iraqi leaders from reaching Tikrit and mounting a potential last stand.

Whether full-fledged Republican Guard combat or guerrilla warfare lies ahead, Capt. Frank Thorp, spokesman at U.S. Central Command, would guarantee only one thing: "This battle definitely isn't over."