Ranged against this concept were three officers with their own ideas about stopping the Germans: Spaatz, Eaker and the RAF's 'Bomber' Harris. They wanted to hit Germany with their heavy bomber fleets until about three weeks before the scheduled landings, and then go after the railroad system. If they could concentrate their bombs on German oil processing and storage facilities, Spaatz argued, it would deny badly needed gasoline to the Luftwaffe and Wehrmacht. The German air force probably would fly to defend these raids, and then could be defeated in the air.
Spaatz also had a further concern; he was on record against mass bombings with their needless casualties, and he saw the strikes against the railroad system as very costly to Belgian and French civilian employees. He wanted no part of that kind of killing for the United States Army Air Forces.
The final choice had to be made by Eisenhower. He took a month to weight the alternatives, then chose the attacks on the transportation system as the better of the two from his viewpoint of assuring a successful invasion. The question of civilian casualties bothered him, also, but in the cold equations of war, that was acceptable as an unpleasant, if necessary, factor.