Thirty Days Has September — James Strauss

This novel is set in the very center of the time of most difficult combat experienced by a United States Marine Corps rifle company.
A second lieutenant, only days from training back in the states is ordered, under fire, to assume command of a company of cast off Marines, all out in the brutal bloody jungle because of the either the worst of luck or the most minor of offenses or infringement. The outnumbered and little supported company is at constant war with vicious units of the North Vietnamese Army while at the same time tearing itself apart every night in deadly encounters between its racially mixed elements.
The enlisted ranks lack all respect for their untested and inexperienced officers, while the officers fight them right back using supporting fires on the enemy as well as their own warring factions. All the men are ruled by terror and fear of the end they know they are not likely to avoid. They are not going home. They are not going to the rear area. And they are only to be kept moving through a valley of death called the A Shau, with only the manner of their passing in question. That same company so riven by internal strife, however, remains frightfully effective in fighting the enemy.
This account of the reality of agonizingly brutal guerrilla combat is written from the perspective of the new lieutenant who sends his last will and testament home to his wife after only three days in combat.