At the start of the Persian Gulf War 5 1/2 years ago, U.S. and allied aircraft rained tons of bombs and missiles on Iraq, rendering its air defense system inoperative for the rest of the conflict.

How is it, then, that the system posed enough of a new threat this week to warrant the launching of 44 U.S. cruise missiles against Iraqi air defense sites, particularly since international trade sanctions were supposed to have inhibited Iraq's ability to rebuild its military machine?

The answer, say experts on Iraq, lies in the fact that President Saddam Hussein's extensive network against air attack was never really obliterated during the Gulf War. A substantial portion of it survived the pummeling by allied aircraft, which had been intent on shutting the system down, not blowing it to pieces.

"We didn't go in there to eviscerate the whole network," said Air Force Col. David Deptula, one of the architects of the air campaign. "The aim was to suppress their defenses. So it's not surprising the Iraqis would have some operable components today."

Saddam's main air defense command centers, located deep underground in hardened bunkers, escaped elimination. And numerous above-ground antennas and radar facilities that were struck have since been repaired from large stocks of spare parts that Iraq had on hand before the war, experts say. The Iraqis also have shown some resourcefulness in obtaining parts from abroad, despite the sanctions, and have gone on to establish new missile sites.

"Much of their antiaircraft equipment is the same used by former Warsaw Pact countries," said a Pentagon official, "and there are lots of spare parts out there now on the open market that the Iraqis could get."

Still, for all its resiliency, the Iraqi system suffers from significant shortcomings, including reliance on outdated Soviet-era technology, a lack of airborne monitoring equipment and a loss of foreign technical assistance.

Iraq's air defense network was patterned after the Soviet model and built largely by the French. Known as KARI (the French name for Iraq, spelled backward), its hub is in Baghdad. The network branches into several regional operations centers, which in turn control tracking centers, aircraft interceptors, surface-to-air missile batteries and antiaircraft guns in their respective regions.

One regional center is in Tallil, among the sites targeted in this week's cruise missile attacks.

The Iraqi system contains considerable redundancy, with one center able to pass control to another if damaged. There is some overlapping coverage by radar dishes. And hundreds of mobile antiaircraft missile launchers can shift locations to set traps for enemy aircraft.

"The Iraqis stopped operating their air defenses after the first few days of fighting in 1991 to spare what they could," said Michael Eisenstadt, who worked on the government's official study of the Gulf War's air campaign and is a senior fellow at the Washington Institute for Near East Studies, a nonprofit research group. "As long as they weren't challenging allied aircraft, they were not targeted. As a result, their losses were reduced, and they emerged with a large part of their system intact."

Little consensus exists among U.S. experts about just what percentage of Iraq's prewar defense system survived, according to Anthony Cordesman of Washington's Center for Strategic and International Studies. But Cordesman estimates Iraq retains at least 380 Soviet-made surface-to-air missile launchers, about 80 French-made Roland units and "large numbers" of portable Soviet-made antiaircraft systems.