What was the significance of the air raid in Dresden? Weren't air raids commonplace during the war?

Up to October 1944, Dresden was not considered a military target. What had changed for it to be the lasting legacy of allied carpet bombing?

Dresden was an old cultural medieval and picturesqe city and had hardly been touched by air raids. For the most part, it was undefended, with no anti-aircraft guns.

In early 1945, German troops were moving through Dresden. This may have been to fight the advancing Soviet Army or to reinforce fighting elsewhere. Dresden was a key transport junction. To Churchill and his war cabinet, this made Dresden a strategic target. Bombing the city might halt the flow of German troops and speed the advance of the Soviet army into Germany. Bombing Dresden might help the Russian war effort.

It was clogged by civilian refugees fleeing the dreaded Red Army.

There may have been another reason for choosing Dresden as a target. Bombing was believed to have an effect on morale. The idea was that civilians would be so traumatised by the continual threat to their lives, that they would stop believing they could win the war and would lose the will to fight. (This had been practised on both sides already. Attacks on places such as London in 1940-41, Coventry in 1941 and Hamburg in 1941 actually made the local people more determined.)

The decision was made at a time when Britain had suffered the Blitz, as well as random bombarding by German rockets, and when Hitler had devastated cities such as Warsaw. British leaders also knew about the Nazi treatment of Jewish people in the concentration camps.

Therefore, in short there were two reason: the train stations were a key hub in the german war effort and Air Marshall "bomber" Harris & others wanted retribution for the bombing of Coventry and other cities. (btw, the trains were up an running several weeks after the attack) The use of incendiaries, the resulting fire storm and death of civilians (mostly women and children), hospitals, churches, schools, etc, made Dresden stand out and on the same pedestal as Hiroshima and Nagasaki.
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just my opinion