Federal authorities have been warning state and local officials since early 2016 that leftist extremists known as “antifa” had become increasingly confrontational and dangerous, so much so that the Department of Homeland Security formally classified their activities as “domestic terrorist violence,” according to interviews and confidential law enforcement documents obtained by POLITICO. Previously unreported documents disclose that by April 2016, authorities believed that “anarchist extremists” were the primary instigators of violence at public rallies against a range of targets. They were blamed by authorities for attacks on the police, government and political institutions, along with symbols of “the capitalist system,”...