John Pershing (1860-1948) Led the American Expeditionary Forces (1st Army) during the First World War. In August, 1918, the supreme commander of Allied forces, Marshal Ferdinand Foch, wanted General Pershing to split his army to aid the British fighting to the north of the American position. Pershing refused, but promised he would support the offensive after he had attacked from his position at Meuse-Argonne. Although not quite to schedule, and using inexperienced American troops, the offensive did break the German line and forced a rout, which effectively ended the fighting on the Western Front and concluded with the Armistice in November 1918.
Had he split his army as suggested, American troops would have gained experience fighting alongside their Allies, but that manoeuvre may have not been the breakthrough that ended the war. Certainly the war would have been prolonged.