| As I know planning and preparing of Operation Overlord started in summer 1943, about battle of Kursk. Of course, allies did not want the USSR defeating the Germans alone because they understood that victory of USSR will result in political changes in territories which Soviets will gain. So allies decided to make their own move and take as much territory in their own control as they could.
All sides waited till opponents and/or concurrents exhaust themselves. USSR waited for the outcome of `phony war` in 1939/1940, so the allies waited the result of USSR-German fight and planned to attack the moment, when opponents will be exhausted and less able to resist and concurrents will be too exhausted to affect the political map after the war. |