Easier to explain: What didn't they compromise?

This looks fairly serious:

Four persons arrested in Los Angeles are part of a Chinese intelligence-gathering ring, federal investigators said, and the suspects caused serious compromises for 15 years to major U.S. weapons systems, including submarines and warships.

U.S. intelligence and security officials said the case remains under investigation but that it could prove to be among the most damaging spy cases since the 1985 one of John A. Walker Jr., who passed Navy communication codes to Moscow for 22 years.

The Los Angeles spy ring has operated since 1990 and has funneled technology and military secrets to China in the form of documents and computer disks, officials close to the case said.

Among the defense systems compromised: The Aegis destroyer, the new Virginia-class submarine.

China covertly obtained the Aegis technology and earlier this year deployed its first Aegis warship, code-named Magic Shield, intelligence officials have said.

The Chinese also obtained sensitive data on U.S. submarines, including classified details related to the new Virginia-class attack submarines.

Officials said based on a preliminary assessment, China now will be able to track U.S. submarines, a compromise that potentially could be devastating if the United States enters a conflict with China in defending Taiwan.

Add this to the theft of America's atomic secrets and the outright sale of missile guidance technology during the Clinton Years of Maximum Profiteering and you have a recipe for major losses in any conflict with China. If having superior technology acts pre-emptively to discourage aggression against you, then Chinese spy efforts have seriously watered down that pre-emptive strength.

Is it enough to give them confidence that they can take us out?

Add this to the equation:

China also is thought to have obtained information from the spy ring that will assist Chinese military development of electromagnetic pulse weapons -- weapons that simulate the electronic shock caused by a nuclear blast -- that disrupt electronics.

It also is thought to have obtained unmanned aerial vehicle technology from the spy ring.
...
Investigators seized hundreds of thousands of pages of documents and computer data from Mr. Chi's home in Downey, Calif., after the arrest.

Mr. Chi and his wife were born in China and are naturalized American citizens. Mr. Tai and his wife are resident aliens who came to the United States from China in May 2001.

The arrests were made after electronic surveillance revealed Mr. Tai and his wife planned to travel to Guangzhou, China, to pass to Chinese officials several CDs that contained Navy weapons data, specifically information on Quiet Electric Drive (QED) systems used in Navy warships, officials said. An FBI affidavit in the case described the QED technology as "extremely sensitive" and banned from export.

The affidavit stated that surveillance showed that Mr. Tai and his wife were "very nervous" and had discussed the risks of carrying the disks to China.

"They were funneling information to 2 PLA," one official said, referring to the military intelligence unit of the People's Liberation Army. "The Chinese now know more about our military than we know about their entire country."

Hit calculate and see what you get.