Authorities have arrested a 20-year-old American after they say he was preparing to detonate a suicide car bomb at an Army base in Kansas. The FBI said that John T. Booker Jr. was acting on behalf of the radical Islamic group ISIS. Booker had attempted to join the Army and then stage an attack on fellow soldiers, but was flagged because of online posts he had previously made about jihad. Last month, Booker met with an undercover FBI informant to make a “martyrdom video” and proclaim his allegiance to ISIS leader Abu Bakr al-Baghdadi. The FBI also said that military personnel at Fort Riley—where Booker planned to carry out the attack—were not at risk at any point …