BAGHDAD, Iraq — Anyone spending Friday night in this sprawling, ancient city on the Tigris River would have found almost no hint that things were much different than they had been a few days before.

The city continues unfazed by the random bomb blasts or small-arms fire that are a daily part of Baghdad life.

And the sound of a U.S. Army helicopter making a routine flight across Baghdad is no more unusual than the sight of U.S. combat vehicles that are everywhere to be seen on the streets and highways of this khaki-walled, bullet-scarred city.

But before Wednesday night, the blasts were typically hit-and-run forays by loyalists of former Iraqi leader Saddam Hussein and other insurgents showing they could strike at will under the very noses of the Americans.

Since Wednesday night, the deep crump of artillery and rapid-fire cannon has been the sound of the Americans hitting back, a shift to a more aggressive posture they’re calling Operation Iron Hammer.

“Our intent is to keep constant pressure on him, to kill or capture him,” said Col. Jackson L. Flake, chief of staff of the 1st Armored Division, “so he’ll feel that there is no safe haven.”

“We are taking firm, hard action against the enemy that doesn’t seem to get the message yet that we’re not leaving, that he can’t win,” Flake said. “Our intent is not to give him any rest.”

“We want to make sure that the Iraqi people understand that the enemy can’t just roam through Baghdad with impunity,” Flake said.

Planners have turned to a mix of weapons and tactics that includes Air Force gunships and Army helicopters, ground troops in combat vehicles, conventional artillery and mortar fire, and such high-tech items as night vision goggles.

“We’re on his home ground, but we have certain technological advantages,” Flake said of the insurgents. “We’re using everything in our kit bag right now.”

Iron Hammer’s first night saw an Air Force AC-130 Spectre gunship fire 18 rounds from 105 mm cannon and 50 rounds from 40 mm machine guns to destroy a warehouse used by insurgents to store bomb-making materials and other weapons, the military said.

Also that first night, U.S. troops faced off against three men firing a mortar. The men fled but left behind a 60 mm mortar tube and nine rounds.

The 1st Armored Division artillery fired 155 mm rounds on the suspected site of a mortar attack on a logistical support area.

And that same night, an Apache helicopter fired at a truck that had been used in a mortar attack on division units, killing two of the 10 occupants and wounding three. Five were taken prisoner. Ground troops later searched the area and found an 82 mm mortar tube, three rifles and three cases of ammunition.

Such strikes have continued, and the distant thud of American rounds detonating can be heard sporadically for hours at a time.

Helicopters can be heard again and again during the night. AC-130 gunships — the H-model Spectre and the U-model Spooky — have also been in the skies.

“This is just the beginning of it,” said Flake. “And over the next several days, weeks if it lasts that long, we’ll do other things.”

But key to mounting Hammer has been intelligence, supplied largely by tips from Iraqi citizens, as well as data pieced together through the military’s own methods of logging and analyzing information, Flake said.

The division had already been keeping track of when and where insurgent attacks had occurred, and analysts had been looking for patterns that might predict the time and place of future attacks.

But several weeks ago, intelligence staffers were directed to “develop a picture of the patterns we had seen over the past two weeks,” Flake said, to help gauge “where was the enemy most likely to fire? When was he most likely to fire? What patterns had he fallen into?”

Then, two deadly attacks set off a surge in tips from Iraqi citizens: the Oct. 26 insurgent rocket attack on Baghdad’s Al Rashid Hotel and the attack a day later on the headquarters of the International Committee of the Red Cross. Deputy Defense Secretary Paul Wolfowitz escaped injury in the hotel attack, but an Army lieutenant colonel was killed.

“After that there was a spike in the walk-ins,” said Capt. David Gercken, the division’s chief spokesman, referring to the number of Iraqis offering tips. “‘Hey, this group of people is carrying out these attacks from near my house.’ So now, instead of the time-date on the map … you have a name.

“We’ve been doing the pattern analysis for a while; shows us where and when … and all of a sudden you get a flood of intelligence. … You combine the two, now you can put the names,” said Gercken.

“Everything that we are doing is intelligence-driven,” Flake said.

“This is just the beginning of Hammer,” said Flake. “It’s not on a time line. It’ll be driven by mission accomplishment. … The other thing we’d like to see accomplished is for the Iraqi people to help us. … We are now getting clear intel from them. They’re helping us identify them, and with their help, we’re able to kill or capture more.”

“I think you’ll see a lot of continued activity as we search out the enemy and take the fight to him,” said Flake.

“If he ventures out now, he runs the risk of being captured or killed.”

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