President Wilson wanted to emphasize the fact that the reasons for American involvement in World War I were different to those of the French and the British. He knew that a large section of American public opinion was still deeply skeptical over the United States' entry into the war, that many Americans didn't see why "their boys" should be risking their necks in yet another bloody dispute between the crowned heads of Europe.

By keeping American forces separate from their allies in the field, Wilson was fleshing out his long-standing conviction that the United States, for one, was genuinely fighting for a noble cause, the cause of freedom, democracy, and national self-determination. This wasn't just an internecine squabble between European aristocrats; there were much bigger issues at stake.

It was hoped that the literal separation of American from other allied troops on the battlefield would help convince the American public that the United States was playing a leading role, not just in determining the general conduct of the war and its overall aims but also by shaping the contours of any future peace settlement.

There were a number of reasons for this.  Here are the two most important reasons that Wilson (and Pershing) had.

• The pride of the US Army.  Putting Americans under the command of the French or the British would have been an insult to the American officers.  It might reduce morale in the Army for this reason.
• The ability of the US to influence the peace.  If the US forces were simply mixed in with the French or British, it might look as if the Americans had not really contributed much to winning the war.  Wilson wanted the American contribution to be very clear.  This would, he hoped, allow him to play a bigger part in shaping the peace treaty after the war ended.