Why was waiting war in Vietnam difficult for US soldiers

Firstly most of the war was fought as a guerrilla war. This is a type of war which conventional forces such as the US army in Vietnam, find notoriously difficult to fight. Conventional forces are easy to identify, guerrillas are not. In Vietnam the Vietcong were peasants by day and guerrillas by night.

Secondly a guerrilla force only needs the support of a small % of the population to sustain its war. In Vietnam although a significant % of the population opposed the VC, their support amongst the population was easily enough to keep the war going.

These factors combined, led to a feeling of optimism at the start to one where an American withdrawal became inevitable.