Hong Kong’s police once had a reputation as Asia’s finest. But Andy Tsang, who is retiring this week as Commissioner after four years on the job, has diminished his force’s reputation for professionalism and impartiality by doing Beijing’s bidding.

Mr. Tsang’s tenure began in 2011, and one of his first acts was to lock down Hong Kong University to protect visiting Chinese Vice Premier (now Premier) Li Keqiang from having to pass by journalists and protesters, some of whom were briefly detained by police.

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