How did some u. s. citizens oppose the vietnam war? many left the u. s. for other countries. many expressed opposition through protest marches. many joined pacifist organizations.

The answer is given by the second sentence. Some U.S citizens opposed the Vietnam war by joining protest marches to show opposition for the said war. Most (but not all) of these protests were usually peaceful. People joined to pressure the United States government to stop the war and to show the despise that they had for the war. This was intensified by the pictures and videos provided by different media outlets. 

i really really really really really really dont know