What U.S. wartime intelligence agency was assisted by the Vietminh?
The Office of Strategic Services (OSS)
During World War 2 the United States created the OSS as a parallel to the British Intelligence Services. OSS operated a small unit in Vietnam under the command Major A.L.A. Patti. This unit had contacts with various anti-Japanese groups including the Vietminh. It held direct meetings with Ho Chi Minh.
The OSS was shut down October 1st 1945, and its work taken over (or not) by the various U.S. Military intelligence groups. The FBI retained authority over Latin America. The Central Intelligence Agency (CIA) was created later (1947).