Police in China have detained a man who asked on social media what law prevented anyone calling self-ruled Taiwan a country, questioning a fundamental principle of China’s sovereignty.

Taiwan is China’s most sensitive diplomatic and political issue.

Beijing views the democratic island as merely a wayward province and it has stepped up a campaign against the island as it tries to assert Chinese sovereignty.

Police in the northeastern city of Maanshan said an 18-year-old unemployed man, identified by the family name Yang, had used his Weibo social media account to post questions on a police Weibo including: “What law says you can’t call Taiwan a country?”

The young man also referenced Japan, saying that Prime Shinzo Abe was his “real father,” the police said in a statement, adding that what he wrote was against the law and “profaned the people’s feelings.”

Yang, who police said had previously been warned for making “bad comments” online, had confessed to his crimes and had been detained on suspicion of “picking quarrels and provoking trouble,” they said.

Defeated Nationalist forces fled to Taiwan in 1949 after losing a civil war with the Communists.

Beijing has begun ordering foreign companies to label Taiwan as part of China on their websites and is excluding Taiwan from as many international forums as it can.