Firebombing is a bombing technique designed to damage a target, generally an urban area, through the use of fire, caused by incendiary devices, rather than from the blast effect of large bombs. Although simple incendiary bombs have been used to destroy buildings since the start of gunpowder warfare, World War II saw the first use of strategic bombing from the air to destroy the ability of the enemy to wage war. The Chinese wartime capital of Chongqing was firebombed by the Japanese starting in early 1939. London, Coventry and many other British cities were firebombed during the Blitz. Most large German cities were extensively firebombed starting in 1942 and almost all large Japanese cities were firebombed during the last six months of World War II. This technique makes use of small incendiary bombs (possibly delivered by a cluster bomb such as the Molotov bread basket). If a fire catches, it could spread, taking in adjacent buildings that would have been largely unaffected by a high explosive bomb. This is a more effective use of the payload that a bomber could carry. The use of incendiaries alone does not generally start uncontrollable fires where the targets are roofed with nonflammable materials such as tiles or slates. The use of a mixture of bombers carrying high explosive bombs, such as the British blockbuster bombs, which blew out windows and roofs and exposed the interior of buildings to the incendiary bombs, are much more effective. Alternatively, a preliminary bombing with conventional bombs can be followed by subsequent attacks by incendiary carrying bombers.