ASHINGTON, Aug. 19 — American forces recently completed the largest joint war-fighting exercise they have ever held, a three-week, $250 million operation that involved 13,500 military and civilian personnel battling in nine live exercise ranges across the United States and in double that many computer simulations.

Results from the mock combat, planned for two years, are expected to shape planning against future adversaries.

As they compiled lessons from the exercise, called Millennium Challenge 2002, officers praised new airborne communications that allowed commanders to stay in touch with farflung fighting forces as never before, even while in transcontinental flight to the battlefield. They also emphasized the importance of combining their destructive power with attacks on computer networks as well as with diplomacy.

Military officials said the troops were also reminded that a wily foe with little to lose retains the historic advantage of the attacker.

Gen. William F. Kernan, head of the United States Joint Forces Command that organized and operated the war game, said the exercise showed the importance of a Standing Joint Force Headquarters to coordinate the efforts of all the armed services during wartime.

The idea, he said, is to avoid "the ad hoc nature" of past wartime command headquarters, thrown together in time of emergency. The standing headquarters would "provide future commanders with a skill set of people with military specialties and a solid appreciation for the complexities of the region," he said.

In the simulation of a Persian Gulf conflict with a foe that might have been Iran or Iraq but was called merely Red, American forces — or Blue — suffered unexpected losses from a sneak attack early in the fighting but then emerged victorious.

In the opening hours of the conflict, the enemy commander was able to deceive American forces by protecting his messages from electronic snooping: he communicated with field officers via motorcycle messengers.

Enemy planes and ships conducted innocent-looking maneuvers for several days in a row, establishing a pattern that did not appear threatening. But the maneuvers left the forces well positioned for a surprise attack, which was initiated using code words during the morning call to prayer from the nation's minarets.

In the computer simulation, an aircraft carrier battle group and ships of a marine Amphibious Ready Group suffered severe damage, according to the enemy chief of state, played by Robert B. Oakley, a former ambassador to Pakistan who also served as the State Department's counterterrorism director.

The American forces "sailed into the gulf assuming they could establish superiority, and disrupt the enemy's command, control and communications with technology," Mr. Oakley said. "But Red decided to surprise them by going first, and used some time-tested techniques for sending messages in ways that can't be picked up electronically or jammed. Red sank a lot of the fleet."

All exercises are to a certain extent artificial, unable to reflect the entire spectrum of wartime, from life-and-death stress on a single soldier to the impact of public opinion as battlefield fortunes wax and wane.

Senior military officers said the value of the exercise was that it required completing a range of missions to test 51 separate military initiatives.

Because many aspects of the war game remain classified, officers would not detail the extent of simulated damage to the fleet, nor say whether the exercise was restarted after the fleet was theoretically hit. Analysis from the the enemy commander, played by Paul Van Riper, a retired Marine Corps lieutenant general, will be incorporated into a final report.

"Both Red and Blue were constrained during the exercise," said one military officer. "You can't stop the entire game when one side gets too clever."

In the end, officials said, the joint American forces — Air Force, Army, Marines, Navy and Special Operations — were declared victorious.

Weapons of mass destruction figured heavily in the exercise, with American forces ordered to attack four sites containing chemical weapons or their delivery systems.

One technological system tested was a complete headquarters-sized communications system that was loaded onto a C-17 cargo plane. Lt. Gen. B. B. Bell, commander for all American forces in the exercise, planned and directed missions while aloft. "We could do what we could do in a large headquarters while we were airborne," General Bell said. "I had all the tools I would normally have in a fixed base."

New communications and tracking systems also allowed commanders to integrate attacks by both Army and Marine Corps ground troops, rather than assign them complementary but separate missions.

"Instead of drawing a line on the map, with the Army on one side and Marines on the other, a commander can now integrate those forces," said Brig. Gen. James B. Smith, who directed the exercise.

General Smith said the value of the exercise, like any war game, would only become clear in the months ahead as the exercise was analyzed and its lessons pushed throughout all the armed services.

"The sunset of this event is the sunrise of a process to work the challenges of integrating operations of the armed services," he said. "If we say this is over, then we're making a huge mistake."