Why did general john j. pershing refuse to allow the aef to serve as "fill in the gaps" of soldiers in europe?

For the answer to the question above, General Pershing also needed to decide whether or not to lend American soldiers to the European allies. When the AEF arrived in Europe, the Allies were exhausted from three years of war. While some American military personnel and European allies wanted Pershing to send American troops to “fill in gaps” in the Allied armies, he established his authority by refusing to do so. He believed the strategy would not work because the training methods used by the U.S. and the Europeans were too different from each other. He also thought that a united American military force would hurt German morale more. Despite opposition, Pershing stood firm in his decision.