The decision you face now, therefore, is crucial. Once large numbers of U.S. troops are committed to direct combat, they will begin to take heavy casualties in a war they are ill-equipped to fight in a non-cooperative if not downright hostile countryside.
Once we suffer large casualties, we will have started a well-nigh irreversible process. Our involvement will be so great that we cannot-without national humiliation-stop short of achieving our complete objectives. 0f the two possibilities I think humiliation would be more likely than the achievement of our objectives-even after we have paid terrible costs.
Memorandum to President Lyndon Johnson from Undersecretary of State George Ball, July 1, 1965
http://www.mtholyoke.edu/acad/intrel/pentagon4/doc260.htm