WASHINGTON - The military has begun a series of exercises with its national missile defense system to move it a step closer to activation. The exercises involve testing crews and activating the network of sensors and command centers to ensure they transfer information properly, Rick Lehner, a spokesman for the Missile Defense Agency, said Friday. "No problems have cropped up," he said. Although five ballistic missile interceptors are in their silos at Fort Greely, Alaska, they are not yet operational because their arming pins have not been removed, so the system is not considered "on alert" yet, Lehner said. The...