For fans of pro tennis, European soccer and British tabloids, the mysterious Twitter account had a lot to offer.

Beginning last year, it retweeted news, most of it in English, about Roger Federer and the Premier League, and it shared juicy clickbait on Zsa Zsa, an English bulldog who won the 2018 World's Ugliest Dog contest.

Then, suddenly, the account began posting, in Chinese, about a different obsession: politics in Hong Kong and mainland China.

By this summer, it had become a foot soldier in a covert campaign to shape people's views about one of the world's biggest political crises.

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