What indicated that the United States had been secretly involved in Vietnam before its official involvement?

The Pentagon Papers were classified government documents that revealed the history of U.S. involvement in Vietnam, including actions taken secretly before the war became official public policy. Therefore, the correct choice is the Pentagon Papers. The other choices refer to a voting-rights amendment, a later policy of reducing U.S. troops, and a cease-fire agreement.

AI-written and checked against the verified answer.