Many Cold War MBTs evolved more or less directly from late World War II medium tank designs. However, in the 1960s and 1970s, a generation of purpose-designed main battle tanks appeared, starting with the British Chieftain tank. These vehicles are less obviously influenced by wartime templates (the Chieftain, for example), weighing as much as a World War II heavy tank and possessing far greater firepower and armour, while retaining the mobility of the previous Centurion design. Similarly, the US M1 Abrams series, the German Leopard 2, the British Challenger 1, French Leclerc and Russian T-90 tanks are all main battle tanks. The defining feature of the main battle tank type is neither its weight, mobility, nor firepower, but instead the idea that only one type of tracked armoured vehicle is required to carry out the roles of breakthrough, exploitation and infantry support.