Nicaragua War White Paper
The Nicaragua White Paper is actually a misnomer, given that the paper in question was focused mainly on El Salvador. The 1981 February 23 State Department paper was entitled "Communist Interference in El Salvador: Documents Demonstrating Communist Support of the Salvadoran Insurgency." The "White Paper" used a variety of intelligence sources, including captured Salvadoran guerrilla documents, to show a flow of material support to the rebels from Nicaragua, Cuba, and a variety of other Communist countries, including the Soviet Union and Vietnam. While a few journalists found some errors in the paper (e.g. Jonathan Kwitny in the Wall Street Journal), most of the claims have been substantiated, using declassified U.S. government documents, exfiltrated KGB documents, and other sources.
See, e.g., Hager, Robert P. "Soviet bloc involvement in the Salvadoran civil war: The US State Department's 1981 “white paper” reconsidered." Communist and Post-Communist Studies 28, no. 4 (1995): 437-470; Onate, Andrea. "The Red Affair: FMLN–Cuban relations during the Salvadoran Civil War, 1981–92." Cold War History 11, no. 2 (2011): 133–154; Andrew, Christopher, and Vasili Mitrokhin. The World Was Going Our Way: The KGB and the Battle for the Third World: Newly Revealed Secrets from the Mitrokhin Archive. Basic books, 2006.