TikTok went dark for American users in an unprecedented display of the U.S.-China divide over technology and national security. The app started halting service Saturday night for 170 million users in its most important market shortly before a law took effect requiring it to shed its Chinese ownership or close in the U.S. It marked the first time the U.S. government has compelled the closure of such a widely used app, and disrupted millions of American businesses and social-media entrepreneurs who use TikTok to connect with customers and fans. TikTok users started seeing a message late Saturday saying, “Sorry, TikTok isn’t available right now.”