A Brooklyn group burned the American flag—but were too scared to show up to their own media circus. This is what happens when you try to host a mass flag burning in Brooklyn: You don't. At least not on the day advertised on social media. Because on Wednesday evening, a horde of patriotic bikers and American-flag-hoisting hotheads crawled out of the city's conservative netherworld and descended on Fort Greene Park—hell-bent on stopping a red, white and blue barbecue they tripped over on Facebook. “We’re here to [expletive] up tiny liberal punks,” one biker reportedly declared. Little did they know the...