1766-1122 BC or 1556-1046 BC. It depends on the source you go with.
It helped because it enabled other fronts to open up because the American Offensive in the area ended with a victory, which meant that they could move to other areas and bother Germans there. They didn't' do this however because the fighting ended since the Germans became aware of their losses and signed an armistice.
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It was known as The Holocaust, which a Holocaust is where one group of people try to eliminate a certain group of people like how the Nazis tried to get rid of the Jewish people.
They wanted to present them as if they were gods. This was quite common in history and is a reason for example why pharaohs got pyramids built in their honor. The giant heads depicted their leaders as gods or someone who is in general larger than life itself. It was a sign of respect and others who would visit would also see this and know who they are dealing with. Such heads can still be found and examined which is important when trying to understand their ancient culture.