How significant was the Battle of the Somme to the Allies winning the war?

The significance of the Battle of Somme to the Allied effort in World War I is subject to great debate by historians.  The battle did have the effect of moving the line about ten miles in the Allies favor.  Some also argue that because of the casualties suffered by Germany at Somme, the Germans were not capable of winning the war moving forward.  These arguments are considered minority opinions.  The consensus opinion is that the entrance of the United States in the conflict was the greatest factor in Allied victory.  On the battlefield, the Battle of the Marne is considered to be an important turning point in the war.  The Battle of the Somme is viewed as a very costly encounter for both sides that claimed over a half million lives and only netted about 12 kilometers of land for the Allies.