"The Vietnam experience left the military leadership feeling that they should advise against involvement in counterinsurgencies unless specific, perhaps unlikely, circumstances obtain -- i.e. domestic public support, the promise of a quick campaign, and freedom to employ whatever force is necessary to achieve rapid victory. In light of such criteria, committing U.S. units to counterinsurgencies appears to be a very problematic proposition, difficult to conclude before domestic support erodes and costly enough to threaten the well-being of all America's military forces (and hence the country's national security), not just those involved in the actual counterinsurgency."

David Howell Petraeus, The American Military and the Lessons of Vietnam: A study of military influence and the use of force in the post-Vietnam era. PhD Dissertation, Princeton University, 1987. Page 305.