Bombing Campaigns were deployed during World War II by both sides (the Allies and the Axis) with the main purpose of attacking and destroying industrial sites, railways, fortified cities, and harbours.
These campaigns involved bombing inhabited areas (with no civilian population) at the beginning, but circumstances becoming what they became during WWII, and both sides launched major bombing air attacks on civilian population.
When war began on the 1st September 1939 the US President Franklin D. Roosevelt asked the nations involved in the conflict (by this point the USA was a neutral state) to keep air bombing concentrated on military targets and avoid civilian populations in defenseless cities; meaning not fortified. France and Britain agreed, with a reply from Britain that the same rule applied to Germany.
When the Germans invaded Poland, their air force (the Luftwaffe) started dropping bombs on civilian populations and military objectives making no differentiation between targets. Germany meanwhile sustained that Warsaw was a fortified city. This air raid destroyed hospitals, refugees’ centers, and residential states. Poland was again bombed on September 13 and 14.
On the other hand, the British RAF (Royal Air Force) was strictly attached to Roosevelt’s appeal and bombarded only military targets and later on banned attacks that would put at risk civilian population; even is enemy was not playing by the same rules, that was until the Rotterdam Blitz where the city was completely devastated by the Luftwaffe.
After this major bombing, both sides started to concentrate in attacking enemies at their most vulnerable point. In 1942, the United States Army Air Force (USAAF) began combing campaigns in the conflict. Following Rotterdam was Hamburg in 1939 and 1945, Tokyo 1944 and 1945, Dresden 1944 and 1945, Berlin 1940 and 1945, London 1940 and 1941, Swinoujscie 1945, Pforzheim 199 and 1945, Darmstadt 1943 and 1944, Kassel 1942 and 1945, Osaka 1945 and finally the dropping of Little Boy (code name for the atomic bomb) in Hiroshima on 6 August and Nagasaki’s Fat Man on 9 August 1945.