In June, hundreds of those businessmen gathered in a hotel ballroom in the southern Chinese city of Shenzhen. They were there to toast the new head of a local Taiwan merchants' association. They sipped baijiu liquor and ate seafood as a troupe performed a traditional lion dance for good luck. An honored guest, senior Communist Party official Li Jiafan, stood to deliver congratulations and a message.

"I urge our Taiwanese friends to continue to work hard in your fields to contribute to the realization of the Chinese dream as soon as possible," said Li, using a nationalist slogan President Xi Jinping has popularized. "The Chinese dream is also the dream of the people on both sides of the Taiwan Strait – our dream of reunification."

Li, who ended his speech to beating drums and loud applause, is a department chief in the Shenzhen arm of the United Front Work Department, an organ of the Communist Party's Central Committee. Its mission: to spread China's influence by ultimately gaining control over a range of groups not affiliated with the party and that are often outside the mainland.

United Front documents reviewed by Reuters, including annual reports, instructional handbooks and internal newsletters, as well as interviews with Chinese and Taiwanese officials reveal the extent to which the agency is engaged in a concerted campaign to thwart any move toward greater independence by Taiwan and ultimately swallow up the self-ruled island of 23 million.