LOS ANGELES -- The celebration at Camp Pendleton was billed as an uplifting event to honor the 250th birthday of the United States Marine Corps and highlight the enduring strength and commitment of the nation’s troops. A simulated beach assault in which artillery was fired from the sand toward the interior of the sprawling base was intended to be a capstone demonstration of the capabilities of the Marine Corps. But the display went awry — and was forced to end early — when an artillery round