With the Allies it had been different.  There had been a French army, a British army, an Italian army, a Belgian army, a Russian army and latest of all an American army.  They had tried to work together in harmony and in the main had done so.  But the British naturally wanted above all to prevent the German armies from reaching the coast where they could threaten England.  The French were especially anxious to prevent Paris being captured.  Either side was reluctant to weaken its own army by sending reinforcements to the other.