He wasn't Supreme Allied Commander over all allied forces, he was Supreme Allied Commander for the Allied Expeditionary Force, which was the forces in NorthWest Europe. Forces in other theaters, such as the Mediterranean , Pacific or the Soviets in Eastern Europe did not report to him.

He wasn't too worried about the Soviets until the two sides started getting close, at which point he started to send messages to the Soviets as to what they were up to and how to avoid shooting at each other when the two sides finally met. These messages initially met with some consternation in Washington, but upon clarification that the communication was purely at the tactical co-ordination level and not political/strategic, they relented.

(See "Crusade in Europe, Eisenhower)