When Taiwanese security personnel detained a suspected spy for China at a top secret military base last month, they may have had a sense of deja vu.

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Air force Capt. Chiang — he was identified only by his surname — was the fourth Taiwanese in only 14 months known to have been picked up on charges of spying for China, from which the island split amid civil war 63 years ago. While Taiwan's Defense Ministry did not disclose details of his alleged offense, his base in the northern part of the island hosts the air force's highly classified radar system and U.S.-made Patriot surface-to-air missiles, both vital to the island's aerial defense.

Chiang's arrest followed that of Maj. Gen. Lo Hsieh-che, who had access to crucial information onTaiwan's U.S.-designed command and control system, and civilian Lai Kun-chieh, who the Defense Ministry says tried without success to inveigle Patriot-related secrets from an unnamed military officer. A fourth alleged spy was detained on non-defense-related charges.

The cases show that China is seeking information about two systems that are integral to Taiwan's defenses and built with sensitive U.S. technology. A major breach could make Taiwan more vulnerable to Chinese attack.

Though relations between the two have warmed in recent years, Beijing has never recanted a vow to retake the island, by force if necessary.

Information about the U.S.-supplied defense systems could also help the People's Liberation Armyunderstand other U.S. defenses. Taiwanese officials, however, say their systems are secure, and U.S. experts say American secrets will remain protected in any case.

The possibility that Taiwan might give up military secrets is certainly a worry for the U.S., its most important foreign partner.

Despite shifting recognition from Taipei to Beijing in 1979, Washington continues to sell the island sophisticated military equipment, and sees it as an element in a string of Asian defense relationships that stretches from South Korea to Australia. Any confirmed leak of U.S. defense secrets from Taiwan to China could undermine U.S. willingness to continue providing military equipment and technology to the island.

"We are concerned whenever this type of incident occurs," a U.S. defense official said in an email response to an Associated Press request for comment on the recent espionage incidents. "However, Taiwan has taken aggressive steps in the last year to protect itself from intelligence threats." The official spoke on condition of anonymity because of the sensitivity of the issue.

China and Taiwan have been spying on each other for decades, and U.S. intelligence agencies have also been active on both sides of the Taiwan Strait, including sharing sensitive mainland-related data with Taiwan. But the recent arrests represent a big upsurge in both the seriousness and quantity of Taiwan spy cases compared with previous years.

At the heart of the China's Taiwan espionage efforts are two systems with substantial U.S. technology — the Lockheed Martin and Raytheon-built Patriot missile defense system and the Lockheed-designed Po Sheng command and control system.

The Patriot uses sophisticated radar to track incoming aerial threats, then launches high-performance missiles to bring them down. The Po Sheng network — the Chinese name means Broad Victory — allows Taiwan's army, air force and navy to exchange battlefield information in real time. That is a big advantage in coordinating responses to the attack China has promised if Taiwan ever moves to make its de facto independence permanent.

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