This building produces and holds one of the major resources of the game: the fuels and lubricants required for the support of infantry and vehicles. It also improves various other structures of your base. The game begins with a level one Fuel Depot. Modernization speeds the production of fuel, and increases the total amount of fuel that you can store.
The Lend-Lease Program was the system through which the United States aided its WWII allies with ammunition, equipment, food, and key raw materials, including petroleum products. The United States delayed its entry into the war, but was quick to share its fuel reserves with its major allies: Britain and the USSR.