One of the major reasons are that those who return from over seas are more knowledgeable of what is truly going on with this government's conflict in other nations. Therefore they are believed to be less likely to help steer the public to follow the propaganda these past and present administrations are constantly telling the public. One way to understand that is to look at what happen to those men returning from the frontline against the Germans during WW 2. The communist regime in Russia didn't trust their own men to not have been turned to sympathize with the Germans against communism.