They must import all of their fuels, which can have undesirable effects on the nations' economies and peace if supplies become costly or if trade embargoes occur. For example, one of the factors that led Japan to bomb Pearl Harbor was the American embargo of oil exports to Japan. Other nations depend entirely on one energy source. The American dependence on foreign oil may be greatly reduced if automobiles and airplanes could be designed to improve fuel efficiency. In comparison, Brazil fuels almost all of its automobiles on less air-polluting alcohol made from sugarcane.
Most coal is derived from the remains of terrestrial plants.
That is, there is not enough oxygen and microorganisms for the dead plants to completely decay. Over time, the partially decayed plants accumulate into peat . Deeply buried peat is first transformed into lignite coal. With greater burial and heat, the lignite loses water and is transformed into subbituminous and then bituminous coals. Major coalfields of the United States are in the Appalachian Mountains, Alaska, Illinois, North Dakota, the Rocky Mountains, and Texas.