1943 saw particularly heavy casualties, both military and civilian, due to long battles and heavy bombing by German and Allied forces. These included:
In Russia, the Battles of Stalingrad, Kursk and Moscow.
In the Atlantic, the continued sinking of Allied convoys by German U-Boats.
In North Africa and the Mediterranean, Allied advances in Tunisia and Sicily.
Allied bombing raids on the Ruhr and Hamburg.
In the Pacific, the Battle of Tarawa.
The mass-murder of Jews was in full progress by 1943, with the liquidation of the Warsaw Ghetto and other Polish ghettos that year.
You can find more statistics and information here