Many cybersecurity experts in the United States and Taiwan worried when Microsoft provided the Chinese government with access to the source code of its Windows operating system in 2003. Their fear was that access to the code would make it easier for China’s People’s Liberation Army (PLA) to develop and carry out new information-warfare techniques.

A recent series of cyberattacks directed against targets in Taiwan and the United States may confirm that “those fears now appear justified,” says a Taiwanese intelligence officer. Taiwan and China regularly engage in low-level information-warfare attacks. But the past few months have seen a noticeable spike in activity. “‘Blitz’ is an accurate description” of the recent attacks, says the Taiwanese security source. “It’s almost like . . . a major cyberwar exercise.”

For many years, observers believed that the balance of cyber war power was tipped in Taiwan’s favor. It has a sophisticated information-warfare program under the control of the Communications, Electronics, and Information Bureau. But China is quickly closing the gap, experts say. In particular, the PLA has been very effective in developing a cadre of young hackers. Information warfare “will be crucial in the opening phases of [any] military offensive against Taiwan, knocking out the communications infrastructures that could be part of the defense strategy,” says Gary Rawnsley, who heads the University of Nottingham’s campus in Ningbo, China, and is a leading expert on cross-strait cyberattacks.

The recent attacks appear to be an attempt by China to take advantage of an ongoing political crisis and a series of government corruption scandals on the island. In addition to “hard” attacks, such as information theft...