It was August 2006 when one of the young Muslim men accused of plotting to kill soldiers at Fort Dix first broached the idea, according to the authorities. Talking to an informer who was secretly taping the exchange, the young man said that he thought he could round up six or seven other men willing to take part, and that a rocket-propelled grenade might be the most effective weapon, the authorities said. And he had one more notion: He wanted the informer to lead the attack, according to a federal complaint. “I am at your services,†the young man is...