WASHINGTON Twitter has added a warning to one of President Donald J. Trump's tweets about protests in Minneapolis, saying it violated the platform's rules about glorifying violence.Trump, a prolific Twitter user, has been at war with the company since earlier this week, when it applied fact checks to two of his tweets about mail-in ballots. Trump added at the end of his tweet, Any difficulty and we will assume control but, when the looting starts, the shooting starts.Twitter did not remove the tweet, saying it had determined it might be in the public interest to have it remain accessible. It does that only for tweets by elected and government officials. A user looking at Trump's timeline would have to click to see the original tweet. On Thursday, Trump targeted Twitter and other social media companies by signing an executive order challenging the lawsuit protections that have served as a bedrock for online free speech.