Explain why public opinion eventually changed to a point where a significant portion of the u.s. population no longer supported u.s. involvement in vietnam

Public Opinion eventually changed when they found out that president Johnson was lying about winning the war
The government went on TV and announced that US almost win the war so we shouldn't be worry, but the next day the entire army lost because of the Tet Offensive done by Ho Chi Minh ( the army were partied because Tet is a biggest holiday in our country and Ho Chi Minh did his surprised attack during this time) 
Later on, the media showed the image of the soldiers are killed and citizens began to oppose the war. The secret Pentagon Papers in the army is showed to the public too, it is the image that so many soldiers had wounded
Also, they have a draft that if you don't go to 4 years college, you go to Viet Nam and fight for the war, many poor people have to become soldiers because they can't afford college
And the Viet Nam War happened at the same time as the Civil War, Cassius Clay, a famous black boxer stood up by saying that his enemy is in the US because they treat him unfairly, not Vietnamese people.

Both speeches are given on the occasion of a great loss of life in battle. Both discuss the bravery of the men who died. They also try to explain that the sacrifice made by the soldiers and their families was worth the price they paid. Both Lincoln and Pericles explain that democracy is important, and dying to protect it is honorable.