Again this weekend, the F-15s and F-16s set off from Incirlik Air Base in Turkey into the skies over Iraq where they dodged antiaircraft fire and retaliated with missiles and bombs in a little-noticed aerial war fought for high stakes and with increasing ferocity this summer.

In what has become an almost daily ritual, the U.S. fighter-bombers run routine patrols of the "no-fly" zone created in northern Iraq at the end of the 1991 Persian Gulf War, and almost always a menace emerges: tiny puffs of smoke signal antiaircraft artillery; or a cockpit alarm announces that radar has "illuminated" the plane; or, more troubling, the aviators detect an Iraqi missile or airplane aloft.

The U.S. aircraft then "respond in self-defense," as the Pentagon puts it. But that doesn't mean they attack the threatening radar or antiaircraft installations. Instead, the payback usually comes hours later and miles away, and it may be delivered by different aircraft.

Last Monday, for example, patrolling aircraft detected antiaircraft fire from three locations. The self-defense response came in the form of precision-guided munitions dropped on a radar site near a major dam. On Tuesday, antiaircraft fire again, and the retaliation was smart bombs on a military installation 28 miles away. On Wednesday, the Iraqis fired antiaircraft guns and later turned on their tracking radar for a moment, provoking bombs on a military depot deep in the desert.

Thursday and Friday were days off. Saturday the routine began again.

Unlike other bombing campaigns against Iraq, which were designed to force a specific concession from Iraqi leader Saddam Hussein, the Clinton administration acknowledges that the current undertaking is reactive by design and has no long-term objectives other than containing Iraq by enforcing long-standing rules for the no-fly zones, one in the north and another in southern Iraq.

"You really can't even call it an 'air campaign' because we are simply responding to Saddam's provocations," said a senior U.S. policymaker. "This was never envisioned as a way to bring about Saddam's overthrow or to take out a significant part of his military establishment."

Administration officials say the bombings help prevent Saddam Hussein from harming his neighbors or internal opposition. "The primary contribution that the patrols make is, they support the containment policy by responding aggressively and quickly to challenges to the patrols," said Pentagon spokesman Kenneth Bacon.

Still, the bombings have become a major military mission. Since Iraqi air defenses began challenging allied planes over the no-fly zones last December, U.S. and British aircraft have flown some 10,000 combat or combat-support sorties, unleashing more than 1,000 bombs and missiles on more than 400 targets, according to military officials.

A recent assessment concluded that the bombings had wrecked as much as a quarter of Iraq's most important air defense weapons--long-range surface-to-air missiles and the radar equipment that guides them, Pentagon officials said. "The degradation of Iraqi air defense assets is a fringe benefit to the way the Iraqis are reacting to our patrols, but that is not the objective of our operations, and Saddam Hussein could stop it tomorrow if he stopped threatening our aircraft," said a senior Pentagon official.

The airstrikes are by far the most forceful action aimed at Iraq. More than a year has passed since Iraq shut down the U.N. weapons inspection program that President Clinton so often proclaimed essential to keeping the peace, and the administration faces an uphill diplomatic effort to impose a new inspection regime. And, nearly three years after Iraqi tanks destroyed an anti-Saddam coalition engineered by the CIA, administration officials acknowledge that within Iraq there is no effective opposition that favors the United States.

Meanwhile, the United States and Britain--Washington's only full partner in the airstrikes--face increasing criticism from some former Gulf War allies. A senior French diplomat said last week that the military situation is "drifting out of control" and that France has "difficulty understanding the U.S.-British airstrikes." Francois Rivasseau, deputy spokesman for the French Foreign Ministry, told reporters that "these raids have now moved quite a long way from their original, basic purpose" of preventing the Iraqi military from using aircraft against armed Iraqi opposition groups--the Kurds in the north and Shiites in the south.

U.S. aircraft pursued that purpose for more than seven years, flying patrols in the no-fly zones virtually unchallenged by Iraq. Then last December, the United States and Britain conducted a four-day bombing campaign, Operation Desert Fox, to punish Iraq for obstructing U.N. inspectors. Soon after the bombing ended, Iraq began to fire surface-to-air missiles at aircraft patrolling the no-fly zones, and the United States started retaliating.

In January and February the action was intense, with Iraqis regularly firing surface-to-air missiles at the patrols and U.S. and British aircraft hitting as many as six targets a day--always radar equipment, missile installations or other sites that were part of Iraq's air defense system. The number of engagements dropped off in the spring as aircraft were shifted to the air campaign against Serbia.

Since the end of the Balkan conflict, the tempo has increased, according to U.S. military officials, with combat sorties now routinely taking place three or four times a week. And, under new rules of engagement approved by Clinton in late June at the request of the military command, pilots have expanded authority "to protect themselves and protect their missions," according to Bacon.

Those pilots are often reservists or National Guard officers who do two-week stints at Incirlik as part of their annual service commitments. When pilots of attack aircraft fly into Iraq, they now regularly carry a list of pre-approved "response options" that can be bombed if the patrol encounters something considered an Iraqi threat--and that happens virtually every time they fly, according to a senior Pentagon official. On many days, the attack planes go directly to a target and drop bombs as a response to a perceived provocation earlier in the day or even the day before, the official said.

According to Iraq, the airstrikes have caused a steady stream of civilian casualties, including more than 20 deaths in August. The Pentagon took the unusual step of countering one such claim last Tuesday with a statement insisting that two civilian deaths in the town of Ba'ashiqah were caused by unexploded Iraqi antiaircraft shells falling back to earth, although the statement offered no supporting evidence.

The administration's sensitivity reflects concern over a U.N. Security Council session next month at which competing proposals on Iraq are to be debated, according to senior officials. The United States is backing a British-Dutch proposal to require Iraq to accept a new weapons inspection program. France has drafted a rival plan that U.S. officials view as too easy on Iraq. French diplomats have argued that a compromise is necessary to break the logjam and bring an end to the U.S.-British bombing "of which the civilian population is the main victim," said France's Rivasseau.

Since the end of the Kosovo air campaign in June, the Clinton administration has been considering expanding the "self-defense" bombing of Iraq to include military targets other than air defense system and widening operations to targets in central Iraq, far from the no-fly zones, according to a senior Pentagon official. Those discussions are at the Cabinet level, but the plans seem likely to be shelved because a more aggressive military effort could arouse opposition at the United Nations and among Arab allies in the Mideast, senior officials said.

The Pentagon opposes any escalation that could put its pilots in danger for uncertain purposes. "We see no reason to go off in another direction or an even slightly different direction," said a senior Defense Department official.

In the meantime the Pentagon has also shelved plans to significantly reduce its deployments around Iraq. A year ago the military hoped to pull back as much as a quarter of the roughly 22,000 personnel in the Persian Gulf region and to respond to future crises with rapid deployments of troops and aircraft. The proposal was put on hold last fall as confrontations developed over Iraq's resistance to U.N. inspections.

With little likelihood of expanding military activity over Iraq or reducing it, the administration appears locked into a game of tit-for-tat that could continue for as long as Saddam Hussein--whatever his motivation--is willing to give the United States reasons to strike back.