focused on firepower and protection more than mobility because tanks worked in intimate liaison with the infantry. There was also the case of the development of a heavy cruiser tank, which focused on armour and firepower to challenge Germany's Tiger and Panther tanks. Classification Tanks have been classified by weight, role, or other criteria, that has changed over time and place. Classification is determined by the prevailing theories of armoured warfare, which have been altered in turn by rapid advances in technology. No one classification system works across all periods or all nations; in particular, weight-based classification is inconsistent between countries and eras. In World War I, the first tank designs focused on crossing wide trenches, requiring very long and large vehicles, such as the British Mark I; these became classified as heavy tanks. Tanks that fulfilled other combat roles were smaller, like the French Renault FT; these were classified as light tanks or tankettes. Many late-war and inter-war tank designs diverged from these according to new, though mostly untried, concepts for future tank roles and tactics. Tank classifications varied considerably according to each nation's own tank development, such as "cavalry tanks", "fast tanks", and "breakthrough tanks". During World War II, many tank concepts were found unsatisfactory and discarded, mostly leaving the more multi-role tanks; these became easier to classify. Tank classes based on weight (and the corresponding transport and logistical needs) led to new definitions of heavy and light tank classes, with medium tanks covering the balance of those between. The British maintained cruiser tanks, focused on speed, and infantry tanks that traded speed for more armour. Tank destroyers are tanks or other armoured fighting vehicles specifically designed to defeat enemy tanks. Assault guns are armoured fighting vehicles that could combine the roles of infantry tanks and tank destroyers. Some tanks were converted to flame tanks, specializing on close-in attacks on enemy strongholds with flamethrowers. As the war went on, tanks tended to become larger and more powerful, shifting some tank classifications and leading to super-heavy tanks. Experience and technology advances during the Cold War continued to consolidate tank roles. With the worldwide adoption of the modern main battle tank designs, which favour a modular universal design, most other classifications are dropped from modern terminology. All main battle tanks tend to have a good balance of speed, armour, and firepower, even while technology continues to improve all three. Being