GOTEMBA, Japan — With a burst of flame, followed by a thunderclap boom that broke the bucolic serenity of the training area in the foothills of Japan’s Mt. Fuji, the first rocket fired by the U.S. Marines from their mobile launcher screamed toward its target, the orange burn of its engine painting a streak across the blue sky.Another five rockets followed in rapid succession, before a second HIMARS truck drove out of its concealed position in a copse of evergreens, fired its salvo of six rockets, then retreated back to cover.The live-fire exercise this week at the U.S. military’s Camp Fuji east…