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While members or ex members of the various branches may differ on my opinion, I feel that the Army has the best program. Over the last couple of decades it has evolved to include extensive language training as well as social sciences, communications and other skills needed to work effectively with an indigenous population.
I have known individuals who were active in this field who also had assignments as Military Attachés in various U.S. embassies.
Again, the program has changed vastly since I was an instructor at the Special Warfare School and looking briefly at the Army's website information, I see that assignment to these schools has become much more selective over time. I recommend that you visit the PSYOP/Civil affairs website for additional information.