As President Trump tries to play hardball in his trade war with Xi Jinping, he faces an adversary who has armed China to play a long and potentially painful game in its contest with the U.S. In the weeks since the U.S. president first slapped sky-high tariffs on China, Beijing has responded with defiance. A spokeswoman for China’s Foreign Ministry posted on X footage from 1953 of Mao Zedong promising to fight to the end against U.S.-led forces in the Korean War. “We are Chinese," she wrote. “We don’t back down." The Mao post and other messages from Beijing highlight...