The M1 Abrams entered service in 1980 as the US Army’s new main battle tank to replace the M60 series. Over time, the M1 evolved from its original 105mm M68 L/52 main gun in common with the earlier M60A1/A3 to the Rheinmetall 120mm smoothbore M256A1 L/44 gun also used by the Leopard 2 tank. In addition to its advanced armor protection and fire control system, the Abrams was the first to employ a gas turbine engine for propulsion in place of the conventional diesel engines used by nearly all other modern tanks. This combination made the Abrams a very agile and formidable weapon system on the battlefield. Since its introduction, the Abrams has been remanufactured into newer and more capable configurations including the M1A1 which introduced the 120mm gun and the M1A1 AIM (Abrams Integrated Management) which modernizes the tank with improved sensors and digital situational awareness. The M1A1 continues as the main battle tank of the US Marine Corps.