Explanation:

The Allied commanders refused to take warning that the bombardment didn't work. After eight days, on July 1, 1916, they ordered the attack. Thousands of British soldiers got out of their trenches and began to advance on the German lines. They were easily gunned down by the Germans. It was the worst day in the history of British warfare. They suffered around 60,000 casualties including 20,000 dead on that first day of battle.
Despite the heavy casualties, the Allies continued to attack. They didn't let up on the attack until November 18. During that time they gained around seven miles of territory, but suffered around 623,000 casualties including 423,000 British and 200,000 French. The Germans had around 500,000 casualties.
With over 1,000,000 total casualties on each side, the Battle of the Somme was one of the bloodiest battles in human history.