The virtual destruction of the Japanese air force by the spring of 1945 meant that the cities of Japan were left exposed to low-altitude bombing.
Japanese cities included some of the most densely populated urban areas on earth. Largely made of wood, the buildings were easy to set ablaze. The closeness of the buildings meant that the fires spread quickly and were difficult to put out.
The Allies thought that the massive civilian casualties that would result from firebombing cities might have a more shattering impact on Japanese morale than the continued military campaign against determined soldiers. Yet the Japanese government still did not surrender.