Around two in the morning on April 13th, an out-of-work car mechanic named Cody Balmer climbed over a metal perimeter fence outside the Pennsylvania governor’s residence. In a backpack, he’d brought a sledgehammer and several Molotov cocktails, which he’d made by pouring gasoline siphoned from a lawnmower into Heineken bottles. It took just a few seconds for Balmer to cross a small, well-kept courtyard and reach the south side of the building, a twenty-nine-thousand-square-foot Georgian mansion overlooking the Susquehanna River. He used the sledgehammer to shatter a first-floor window of the state dining room, which housed a Steinway piano, then...