BEIJING—Chinese President Xi Jinping presided over a grandiose military parade marking the 70th anniversary of Communist rule, a projection of strength as the country wrestles with a challenge from President Trump, while Hong Kong braced for another round of anti-Beijing protests.

“There isn’t any force able to shake our great motherland’s status,” Mr. Xi declared from the podium in Tiananmen Square, where Mao Zedong proclaimed the founding of the People’s Republic of China seven decades ago. “There isn’t any force able to...