Moving American ground troops through Iraq depends on resupply, and this depends on the U.S. Army's 16,000 HEMTT (Heavy Expanded Mobility Tactical Truck, also known as the M-977) cross country trucks. If these can keep up with the advancing combat vehicles, especially with fuel resupply, the advance will continue moving. The 8x8 trucks are built for cross country operations and were able to keep up with armored formations during the 1991 war. Most of the 13,000 HEMTT's the army has carry tons of cargo or 2500 gallons of fuel. The HEMTT can also tow a trailer carrying another ten tons of load. There are also some wrecker and tractor models. HEMTT began to enter service in the late 1980s, and some were used during the 1991 Gulf War. The latest version of HEMTT, introduced in the early 1990s and used extensively in the Balkans, is the palletized load system (PLS) that allows 16.5 tons of cargo to be quickly unloaded by the driver. There are 3,000 of these. There are also 2,000 HETs (Heavy Equipment Transporters) that can haul 70 ton tanks on a flat bed. Supplies can also be moved on a HET. In combat situations, supplies are loaded on "flat racks" (removable 8x20 foot cargo beds), which are then loaded on HEMTT and quickly dropped off, with the flat rack, in the combat zone. The army has 16,000 of the flat racks, which are reused. The HEMTTs cost about $200,000 each (depending on special equipment and configuration.) Starting in 2004, the HEMTT fleet will be "remanufactured" (at about half their original cost) to give the trucks another ten years of life. The U.S. Marine Corps uses a vehicle similar to the HEMTT for moving supplies.