The Visitors 1972 ( 6.125)

Bill, Martha and their little child Hal are spending a quiet winter Sunday in their cosy house when they get an unexpected visit from Mike Nickerson and Tony Rodriguez. Mike and Tony are old acquaintances of Bill; a few years back, in Vietnam, they were in the same platoon. They also became opposed parties in a court martial - for a reason that Bill never explained to Martha. What happened in Vietnam, and what is the reason for the presence of Mike and Tony ?