Actually, believe it or not, I think what happened was United States banned the shipment of oil to Japan because of their aggression in China. At the time, United States was one of the biggest supplier of oil.
Japan considered that an act of war and bombed Pearl Harbor to give themselves time to build up a defensive wall of islands surrounding Japan.
I still have the impression that it was metal.......however, either case presents a debate as to what constitutes an act of war......
Regards......HRAWH.