Answer: Morale, Terrain, Traps, Weaponry and flashbacks .
Explanation: Morale is a important factor in any army. So at the start of the war most of our solders thought it would be amazing to fight in a war. While the South at the being was already lone on the morale specially after the 1963 rebellion. Than later in the war our morale was low because of the drafts, and anti-war movement. Than the Terrain was bad. The jungles were full of North Vietnam troops. And if they were well hidden they can cause heavy loses to our troops. Traps like bamboo spikes, snake traps. And the tunnels were full of traps. Like made in a way to flood when US troops would be in their looking for enemy. Weaponry. Since a nice amount of the combat happened in areas were tanks could not travel since in the past few wars we used tanks a lots. And Vets and people deployed would suffer from flashback and would often get flashback from combat,and if the terrain in Vietnam was the same in the States it could give them flashback.These are factors that made the war difficult for South Vietnam and the US troops.