Since 2015, the Defense Department has tapped surpluses from its fuel accounts for $80 million to train Syrian rebels, $450 million to shore up a prescription-drug program riddled with fraud and $1.4 billion to cover unanticipated expenses from the war in Afghanistan, according to military accounting records.

The Pentagon has amassed the extra cash by billing the armed forces for fuel at rates often much higher — sometimes $1 per gallon or more — than what commercial airlines paid for jet fuel on the open market.

Under a bureaucracy that dates to World War II, the Defense Department purchases all of its fuel centrally and then resells it at a fixed price to the Air Force, Navy, Army, Marine Corps and other customers, who pay for it out of their own budgets. The system is intended to reduce duplication and promote efficiency.

The Defense Department is the largest single consumer of fuel in the world. Each year, it buys about 100 million barrels, or 4.2 billion gallons, of refined petroleum for its aircraft, warships, tanks and other machines.