The Tet Offensive was what broke the will of the American People at home. It was a war that deeply divided the American public especially those who would be sent over to fight. Many of them came to Canada (where I live) and many others made a production out of not going. They simply did not agree with the premise the war was based on.
The Tet offense showed the North Vietnamese army to be adaptable, resourceful, determined and very well able to fight a war on their own terms. It broke the will of the American people when the body bags started arriving home.