What is the main conflict in The Killing Zone: My Life in the Vietnam War by Frederick Downs?

The author, Lt. Frederick Downs is surprised that the lights are blazing at the airfield when he arrives in Vietnam, on September 8th, 1967. It does not look like an airfield in the middle of a war zone. Still, there is a foreboding notion that there is a war out there, and men will die, be wounded and be crippled. The author, Lt. Frederick Downs, is a Lieutenant, fresh out of OCS, Officer Candidate School.