BEIJING—Four months before Xi Jinping became China’s leader in late 2012, he issued a Communist Party edict on Hong Kong that reverberates today.

As head of a party committee overseeing the former British colony, Mr. Xi ordered officials to wage combat against what he saw as a growing separatist movement, according to a retired senior official responsible for Hong Kong affairs. “We must dare to struggle and be good at fighting,” the retired official said in describing Mr. Xi’s approach.

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