The Revolutionary War and the Civil War both included and affected the totality of Americans. They were wars fought on America's own soil for America's own national interests.
The Vietnam War is an example of a limited war because it was a commitment of a portion of the US military for a specific purpose elsewhere in the world. It grew to be extremely large in scope and involved an enormous expense of dollars and lives, but it still was "limited" in the sense that it did not involve all Americans at home.
The United States got involved in fighting in Vietnam because it feared that if Vietnam fell to the communists, other countries in Southeast Asia would follow. This was known as "domino theory" -- that the spread of communism in Vietnam would result in other countries in the region falling like dominoes to the influence of communism.
