Travels in southern Iraq yesterday yielded a mosaic of images that illustrate the audacity of the American-led military campaign, and also some of its potential vulnerabilities.

American troops are racing toward Baghdad with astonishing speed. Yet in addition to delivering "shock and awe" to the Iraqis, the Americans are getting a few surprises of their own -- including tough battles to secure several southern Iraqi cities that were expected by some military planners to be easy victories.

The campaign's stunning military might was clear in the convoy of British and American armored vehicles that curved up the arc of the main Baghdad highway under an ash-blue afternoon sky. This mobile firepower is as intimidating, in its way, as the aerial bombardments over Baghdad the last two nights, and the tanks have moved toward the Iraqi capital with astonishing speed.

Yet part of this massive column was held up for a time yesterday afternoon by the discovery that Highway 8, the four-lane road connecting Baghdad with the south, had been mined -- briefly forcing some vehicles onto smaller and slower secondary roads nearby. One mine, probably of crude design and costing only a few dollars, was said by a British soldier to have exploded under an armored personnel carrier.

There were moving scenes of grateful Iraqis waving to the American troops who had come to liberate them from Saddam Hussein. A farmer named Haider told me the American invasion was "the best celebration" he'd had since the Baath Party seized control of Iraq more than three decades ago. Two of Haider's brothers have been executed for alleged crimes against the regime, he said.

"It's a criminal regime, and they execute everyone, for one word, even," said a 14-year-old boy named Mohammed, who said one of his brothers had been executed. Asked what kind of government he wanted to see in the future, a jubilant farmer named Salem Muhsen answered: "Anything but Saddam's terrorism." Two of his cousins had been executed, he said, for the crime of traveling to Kuwait to sell their vegetables.

This was the face of Iraqi liberation. The farmers had gathered at the Safwan intersection, the same spot where the peace treaty ending the 1991 Gulf War had been signed.

Yet soon after these happy sentiments were voiced, a battered white Toyota pickup arrived at the intersection bearing the bodies of two men who apparently died during American attacks. Cradling them was a woman dressed in black, who said the men were her father and brother. She wailed inconsolably, denouncing the American invaders. Her anguish was so intense that when she arrived in the nearby town of Safwan, reporters there said the anti-American mood turned ugly.

You could see a lot of bitter faces in southern Iraq yesterday -- as people watched their country devoured by this immense military machine. And southern Iraq was supposed to be the easy part.

This civilian anger is compounded by the fact that little humanitarian aid seems to be available yet for the victims of war. They were literally begging for help yesterday from journalists -- who were the only Americans not in uniform they could see.

U.S. strategists had assumed that in their race toward Baghdad, they could initially bypass many of the smaller towns and cities of southern Iraq. As Marine Staff Sgt. Brian Koenig put it, relaxing in the shade of his amphibious vehicle parked along the road north toward Nasiriyah and Baghdad, each U.S. combat team has a "bypass criterion" that allows it to focus on the biggest battles, and leave the little ones for later.

But this confidence seemed misplaced yesterday. Despite reports about the quick capture Friday of the port city of Umm Qasr on the gulf, for example, a trip down the Umm Qasr road made clear that the city was far from secure, 24 hours after the coalition's supposed victory. American control was said to extend only to part of the city, and armed Iraqi soldiers were reported to have donned civilian clothes and slipped into the local population.

Similarly, a tough battle was being fought for the city of Basra, with Iraqi forces showing stronger than expected resistance. Some analysts had expected Basra to be a cakewalk for the coalition, because its largely Shiite population dislikes Hussein. Basra will doubtless fall soon, but probably not in the act of anti-Hussein insurgency some American planners may have expected. The danger is that it will feel like a defeated city, rather than a liberated one.

Will the snags in the south be repeated on a larger and more dangerous scale in the decisive battle for Baghdad? Will America's attempts to destroy Iraqi resolve instead stiffen it, as so often happens in war? America's military challenge, in the remaining days or weeks of battle, will be to avoid turning popular hatred of Hussein into anger against the "shock and awe" military colossus that has come to topple him.