Complex sentence: Mexico was producing 55 million barrels of petroleum by 1917, since 75 percent of the fuel used by the British fleet came from Mexico, Carranza gave the order to set in fire the oil fields in case of an American invasion.
Split into simpler sentences: Mexico was producing 55 million barrels of petroleum by 1917. Because 75 percent of the fuel used by the British fleet came from Mexico, Carranza gave the order to set fire to the oil fields in case of an American invasion.