Perhaps you should try some spelling variations:
"oponen of U.S. involvement in the Vietnam War" » "ponen of U.S. involvement in the Vietnam War", "opone of U.S. involvement in the Vietnam War", "componen of U.S. involvement in the Vietnam War"
"proponen of U.S. involvement in the Vietnam War" » "prponen of U.S. involvement in the Vietnam War", "propone of U.S. involvement in the Vietnam War", "prcomponen of U.S. involvement in the Vietnam War"

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: (((("oponen of U.S. involvement in the Vietnam War") OR ("proponen of U.S. involvement in the Vietnam War"))) OR ("component of U.S. involvement in the Vietnam War"))~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"oponen of U.S. involvement in the Vietnam War\") OR (\"proponen of U.S. involvement in the Vietnam War\"))) OR (\"component of U.S. involvement in the Vietnam War\"))".
• Removing quotes may allow a broader search: (((( oponen of U.S. involvement in the Vietnam War ) OR ( proponen of U.S. involvement in the Vietnam War ))) OR ( component of U.S. involvement in the Vietnam War )).
• Adding a wildcard symbol may retrieve word variants: (((("oponen of U.S. involvement in the Vietnam War") OR ("proponen of U.S. involvement in the Vietnam War"))) OR ("component of U.S. involvement in the Vietnam War"))*.