The landmark antitrust lawsuits, announced by the Federal Trade Commission and New York Attorney General Letitia James, mark the second major government offensive this year against seemingly untouchable tech behemoths. The Justice Department sued Google in October for abusing its dominance in online search and advertising—the government’s most significant attempt to buttress competition since its historic case against Microsoft two decades ago. Amazon and Apple also have been under investigation in Congress and by federal authorities for alleged anticompetitive conduct.