Facebook has shut down a sophisticated disinformation operation on its platform that engaged in divisive messaging ahead of the U.S. midterm elections, the company said Tuesday, an escalation of what a top executive described as an “arms race” to manipulate the public using its tools. Facebook said it discovered 32 false pages and profiles that were created between March 2017 and this May, which lured 290,000 people with ads, events and regular posts on topics such as race, fascism and feminism — and sought to stir opposition to President Trump.