Why did Americans support the Korean War but oppose the Vietnam War?
In Korea, American troops were attacked by North Korea while in Vietnam, America inserted itself into the conflict.
The Korean War started in June 1950 when North Korean troops invaded the south in what North Korean deemed a unification program.
The North Korean army attacked the U.S. 2nd Infantry Division and the 1st Marine division pushing them down the Korean peninsula to Pusan. America rightly saw this as an act of aggression similar to the attack on Pearl Harbor.
In Vietnam, America assumed the role of protector of freedom for the South Vietnamese. At the time a theory called "the domino effect" was presented to the American public as reason for our sending troops to Vietnam in the first place. The domino effect was the idea that if America did not stop communist aggression, particularly that of the communist Chinese who were supporting the North Vietnamese government, then once they took over Vietnam they would turn their attention of other adjoining countries and take them over as well.
The example used to illustrate that was how the USSR had taken over the governments of eastern Europe and put up the "Iron Curtain."
From 1965 through 1970, American sent hundred of thousands of troops to Vietnam, bombed North Vietnam using B-52 bombers and by 1970 there were no signs of success or that victory could ever be attained.
Starting around 1967 American college student became very vocal opponents to the war and constantly challenged the Johnson and Nixon administrations in there being a good reason for our being there at all.
As the death toll rose, the tolerance of the average American to fight such a war dwindled. By the early 70s the Nixon administration was desperately looking for an exit strategy.