A recently-released FBI report about the compromising ties between a Chinese-American Mata Hari and her FBI-agent lover is a stark reminder that after terrorism, the greatest threat to our national security at home is espionage.

According to the U.S. government, spies from more than 140 (of 191) nations are working overtime in our exceedingly open society to pinch U.S. defense and commercial secrets at a clip not seen since the Cold War ended.

The FBI report, which details the 20-year "relationship" between Katrina Leung, an informant on Chinese intelligence activities in the U.S., and FBI counterintelligence (CI) agent James Smith, shows why America better keep its eye squarely on the spy threat.

In fact, experts say we're facing the biggest foreign intelligence challenge since the fall of the Berlin Wall. Foreign spies are aggressively collecting military and high-tech secrets using both "old school" and new spy tradecraft.

Foreign spy services are deviating from the traditional practice of placing operatives under official cover in their embassies, consulates or trade missions around the United States. According to the FBI, today's foreign 007's are tasking defense attaches, émigrés or visiting students, businessmen, scientists and researchers to respond to specific information requests or exploit "targets of opportunity" they come across.

American high-tech industries are a key target. Every year, economic espionage costs American businesses billions of dollars. Spies recruit company insiders, form joint ventures, and even engage in "dumpster diving" for discarded proprietary data.

While the U.S. is targeted for intelligence collection by many nations, the top 10 countries probably account for 60 percent of the counterintelligence threat, says the U.S. government's National Counterintelligence Executive office.

China, by far, constitutes the greatest CI threat today, according to the FBI. As many as 3,500 Chinese "front companies" operate here with the expressed purpose of gathering intelligence, especially highly-prized information technology.

The FBI claims that the number of Chinese CI cases in Silicon Valley increases by 20 percent to 30 percent every year. But Beijing also uses espionage to support its unprecedented military buildup.

Just last month, a Taiwanese citizen pled guilty to spying for Chinese intelligence in the United States. He brazenly sought to buy -- and illegally export -- cruise/air-air missiles and helicopter/fighter aircraft parts to China.

Russia is no post-Cold War pal on the Spy vs. Spy front, either. Moscow ranks as no. 2 on the CI threat list. Former KGB colonel -- and current Russian president -- Vladimir Putin keeps his former comrades plenty busy.

Russian spies aren't as taken by high-tech industrial espionage as the Chinese. Not surprisingly, Russian shpioni are more interested in gathering Pentagon/military secrets and "Inside the Beltway" chit-chat for their Kremlin masters.

Cuban 007's are here, too. Havana concentrates its "defensive" intelligence efforts on penetrating the Cuban-American communities in the States to ensure they don't get stung by another Bay of Pigs-style invasion.

Iran's secret service has similar counterrevolution concerns. But while Tehran is focused on Washington's deliberations over its nuclear program, it's also scrounging for embargoed spare parts for its Shah-era, U.S.-made fighter and cargo planes.

Moreover, several Iranian "diplomats" assigned to their U.N. mission were expelled from the U.S. over the last few years for suspicious activities, including "video-casing" New York City tourist and transportation sites.

We can only assume that non-state actors are gathering intelligence here, too. Hezbollah and Hamas undoubtedly are collecting information for terrorist strikes against U.S. targets in case we take military action against their sponsor, Iran.

Spies also focus on members of Congress and staff on Capitol Hill. They see them not only as a source of insider "skinny" on legislative branch deliberations, but as candidates for "influence operations," too.

Considering the ongoing threat of preventing another act of terrorism here at home--and the continuing challenge of law enforcement--adjusting to the growing threat of counterintelligence is no small task for the FBI.

The FBI is doing its best to counter the growing CI problem, including putting "spy catchers" in all 56 field offices. In addition, the Bureau has increased the number of counterintelligence officers in its ranks.

Equally important, the FBI has expanded its counterintelligence cooperation with industry partners, especially with the four major defense contractors, to facilitate spy spotting by the Hoover Gang.

But, unfortunately, that may not be enough. We've clearly got to do more to prevent foreign spies from nicking sensitive American information for ill-gotten commercial, military -- or worse yet -- terrorist gain.

Peter Brookes, a senior fellow at The Heritage Foundation, is the author of "A Devil's Triangle: Terrorism, WMD and Rogue States."