In late December, Walker returned to London. There was no welcome home party waiting to greet him. Instead, there were three police officers at the airport who swiftly arrested him. The officers took him into custody, interrogated him, searched his apartment, and confiscated his laptop and notebooks. After risking his life to fight against the Islamic State, Walker was charged under British counterterrorism laws — not directly because of his activities in Syria, but because the police had found in a drawer under his bed a partial copy of the infamous “Anarchist Cookbook,” a DIY explosives guide published in 1971 that has sold more than 2 million copies worldwide.