The Aug. 5 ceremony to open the pipeline underscored how much Kinmen, home to about 130,000 people, has been pulled into the orbit of China, whose ruling Communist Party has never controlled Taiwan and wants to annex it.

Liu Jieyi, the director of Beijing’s Taiwan Affairs Office, used his speech at the ceremony on the island to demand that self-governing, democratic Taiwan accept the “One China” policy, which declares that Taiwan and China are part of the same country.

“The vast populace of Taiwan will certainly make the correct choice,” Mr. Liu said.

He almost certainly wouldn’t have made such a speech on Taiwan’s main island, where suspicion of China runs high. When Mr. Liu’s predecessor toured Taiwan in 2014, he was met with protests in multiple cities and his car was splashed with paint.

Wang Ting-yu, a Taiwanese lawmaker with the Democratic Progressive Party, said the freedom and democracy enjoyed in Kinmen made it unlikely that its residents would want to be part of authoritarian China. But he said China’s ruling Communist Party had enjoyed some success on the island with so-called United Front tactics, under which it works with non-Communist groups to achieve its goals.