Following the Tet Offensive and the decreasing support among the U.S. public for the war, U.S. forces began a period of morale collapse, disillusionment and disobedience. At home, desertion rates quadrupled from 1966 levels. Among the enlisted, only 2.5% chose infantry combat positions in 1969–1970. ROTC enrollment decreased from 191,749 in 1966 to 72,459 by 1971, and reached an all-time low of 33,220 in 1974, depriving U.S. forces of much-needed military leadership.

Open refusal to engage in patrols or carry out orders and disobedience began to emerge during this period, with one notable case of an entire company refusing orders to engage or carry out operations. Unit cohesion began to dissipate and focused on minimising contact with Viet Cong and PAVN. A practice known as "sand-bagging" started occurring, where units ordered to go on patrol would go into the country-side, find a site out of view from superiors and rest while radioing in false coordinates and unit reports.  Drug usage increased rapidly among U.S. forces during this period, as 30% of U.S. troops regularly used marijuana, while a House subcommittee found 10–15% of U.S. troops in Vietnam regularly used high-grade heroin. From 1969 on, search-and-destroy operations became referred to as "search and evade" or "search and avoid" operations, falsifying battle reports while avoiding guerrilla fighters. A total of 900 fragging and suspected fragging incidents were investigated, most occurring between 1969 and 1971.  In 1969, field-performance of the U.S. Forces was characterised by lowered morale, lack of motivation, and poor leadership. The significant decline in U.S. morale was demonstrated by the Battle of FSB Mary Ann in March 1971, in which a sapper attack inflicted serious losses on the U.S. defenders.William Westmoreland, no longer in command but tasked with investigation of the failure, cited a clear dereliction of duty, lax defensive postures and lack of officers in charge as its cause.