Three U.S aircraft carriers, 22,000 troops and about 290 aircraft are participating in a repeat of Valiant Shield off Guam, a big interoperability exercise that was first held last year.

The exercise, which kicked off yesterday and will be held over eight days, "tests the military's ability to rapidly bring together joint forces in response to any regional contingency," the Navy said.

Three destroyers, a cruiser and submarines from Pearl Harbor are among the more than two dozen ships taking part in Valiant Shield. Carrier port visits are likely after the exercise.

The Air Force is bringing in fighters and B-52 bombers that will participate in a "sink exercise" using satellite-guided bombs. The Marine Corps and Coast Guard also are participating in the war games.

Last summer's gathering of carriers was the first time in two decades that three flattops operated together during training in the Pacific.

The carriers John C. Stennis, Kitty Hawk and Nimitz are participating in exercises including anti-submarine warfare to guard against a growing tide of quiet diesel-electric submarines.

The Navy said dozens of countries, including North Korea and Iran, have diesel subs, and more than 180 foreign submarines operate in the Pacific within reach of critical choke points and navigational sea lanes.

Lt. Cmdr. John Filostrat, a U.S. Pacific Fleet spokesman, said anti-submarine warfare is a big emphasis.

"Anti-submarine warfare is one of the top warfighting priorities for Pacific Fleet, and that is going to be a focus for the three carrier strike groups," he said.

Last October, a Chinese Song-class diesel-powered attack submarine surfaced within sight of the Kitty Hawk strike group in the East China Sea near Okinawa.

U.S. Pacific Command is seeking a better understanding of China's intentions as the Communist nation builds up its military, including its submarine force, to prevent a miscalculation.

In last summer's Valiant Shield, the destroyer USS John S. McCain "sunk" five simulated hostile submarines in the first few days of the exercise.

Following ship-based sonar identification of an enemy submarine, SH-60F Seahawk helicopters can use "dipping sonar" to pinpoint the location and simulate attacking a sub with Mark 46 torpedoes.

Six unidentified U.S. submarines are participating in the exercise.

Although China was among foreign nations that were invited last summer to observe Valiant Shield, no such invitation was made this year.

"We wanted to focus solely on the exercise (this year without foreign observers), and it was going to be similar to last year's," Filostrat said.

The exercise will allow the U.S. forces to "focus on an integrated joint training environment within a real-world scenario," Filostrat added.

Part of the exercise includes time and space separations, with maritime force commander in San Diego, the air component command with the 13th Air Force at Hickam Air Force Base, and the war games themselves near Guam.

Command and control, search and rescue, maritime interdiction and intelligence-surveillance-reconnaissance also will be part of the training.

Reach William Cole at wcole@honoluluadvertiser.com.