Former U.S. Ambassador Eric John warned of China’s assertive diplomacy in a confidential cable before he left his post in Bangkok in September.

Mr. John pointed out “China’s sustained, successful efforts to court Southeast Asia and Thailand” in his report to Secretary of State Hillary Rodham Clinton.

Thai “government officials and academics sympathetic to the U.S. see the dynamic of China rising — and the U.S. receding — likely to continue, unless the U.S. takes more vigorous action to follow-up with sustained efforts to engage on issues that matter to the Thai [people] and the region, not just what is perceived as the U.S.’s own agenda,” Mr. John said in the cable released by the anti-secrecy website WikiLeaks.

Thailand and the United States established diplomatic relations in 1833.

Mr. John also said China is competing strongly with the United States on the cultural front, noting “many Chinese diplomats [who] are fully fluent” in the Thai language.

Chinese Ambassador Guan Mu is one of those Thai-speaking diplomats. He sent 17 years of his diplomatic career in Thailand, rising to the post of ambassador in 2009. Mr. Mu frequently appears on Thai television.

Mr. John, who speaks Korean and Vietnamese, sent three years in Bangkok; and the new U.S. ambassador, Kristie Kenney, who speaks French and Spanish, arrived in January.

China also has marshaled a parade of high-level visitors to Thailand in recent years.

In 2009, alone, Premier Wen Jiabao and Foreign Minister Yang Jiechi visited Bangkok for a meetings with Asian leaders, while Defense Minister Gen. Liang Guanglie came for military talks.

Thai Princess Maha Chakri Sirindhorn visited China five times that year, while Thai Prime Minister Abhisit Vejjajiva and Foreign Minister Kasit Piromya also held talks in Beijing.

China also avoids criticizing Thailand.

In 2006, when Thailand’s military staged a coup and toppled the prime minister, Washington suspended $24 million in military assistance and restricted high-level meetings.

China described the coup as Bangkok’s internal affair, gave Thailand $49 million in military aid and increased the number of exchange students at Chinese and Thai military staff colleges. Beijing also persuaded the Thai military to participate in yearly, small-scale special forces joint exercises.

Last year, Chinese and Thai special forces held a 15-day joint antiterrorism drill, and more than 100 Chinese marines from an amphibious special warfare held exercises with their Thai counterparts.

But China has sold inferior weaponry to Thailand, making some Thai military officials wary of becoming dependent on Chinese supplies.

Thailand’s incoming prime minister, Yingluck Shinawatra, appears eager to expand business ties with China. She favors a Chinese proposal to construct high-speed trains and replace Thailand’s decrepit railway.

In January, Chinese investors began building a $1.5 billion China City Complex near Bangkok to manufacture clothing, household items and other goods.

Chinese migrants have been settling in Thailand for generations, arriving through Laos and across the Mekong River or, more often, by sea from China’s southeast coastal towns to Bangkok.

Today many ethnic Chinese hold some of Thailand’s highest political, economic, military and cultural positions.

Chinese faces, fashions and symbols are promoted in Thai advertisements and pop culture as badges of financial success.

Several top Thai corporations, meanwhile, are trying to make profits by investing in China and hoping to copy the success of Thailand’s Charoen Pokphand (CP) Group, which opened a chicken-processing plant in China in 1979.

The CP Groupt also invested in huge supermarkets, entertainment complexes and other industries.

Thai exporters use Bangkok’s port to ship goods along the Gulf of Thailand and South China Sea into Hong Kong.

China, however, produces much larger and more diversified foods for export into Thailand, threatening local producers.

“People in Thailand are worried,” one former Thai diplomat told The Washington Times. “China’s economy is so big, and ours is so small, that we cannot compete with all the Chinese things being sold here.”

Another official expressed alarm over China’s growing economic clout.

“China will own us!” she declared. “Thailand will be like a vassal of China.”

Both asked not to be identified so they could talk candidly about China’s increasing influence in Thailand.