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The gloves are off. For years, the White House has danced around the sensitive topic of Chinese hacking into American computer systems that is believed to have compromised everything from electrical grids to the email accounts of researchers focusing on China’s human-rights record. Public finger-pointing at Chinese hackers has been left largely to the American legislative branch or to private Western cyber-security firms like Mandiant or McAfee, which have produced reports linking the Chinese military to online espionage. Even when U.S. President Barack Obama warned of the dangers of cyberwarfare in his State of the Union Address last month and then issued an executive order to protect America’s online borders, he declined to specifically name China as an...