The Jungles of Vietnam and the Vietnam War
Most of Platoon takes place in the jungles of Vietnam. Okay, technically it's the Philippines where the film was shot, but it's supposed to be Vietnam. The only respite from all that sweaty foliage and underground darkness is when the boys head up in a helicopter—and even that's scary.
The soldiers in the film spend 90% of their time trekking through dense, wet, mosquito-ridden jungles replete with hidden enemy bunkers and equally dangerous jungle creatures (in one scene, Taylor notices a very eerie-looking snake slithering between his legs). While the jungles of Vietnam are where American troops conducted most of their campaigns in the late 60s and early 70s, the jungle isn't only important for reasons of historical accuracy.
The wet, dense foliage that the soldiers have such a difficult time navigating is richly symbolic as well. The fact that the soldiers often can't see more than a few feet in front of them, and the fact that dangerous jungle animals occasionally appear with little nor no warning (Taylor and the snake, for example) is not only a fitting metaphor for the whole Vietnam War (a sly, secretive, and seemingly invisible enemy), but for the larger international political problems that led to the war in the first place. Hindsight is always 20/20, but back in the mid 60's, U.S. politicians, generals, and policy makers really couldn't see too far ahead, just like the soldiers on the front lines.
A Little Background for You
The spread of communism (the domino effect) was frightening enough to demand military action...at least, according to the higher ups in Washington in the 60s. Just as the soldiers don't really know what's in front of them in the jungle, or what kind of enemy will pop up in front of them in a well-hidden bunker, it was pretty tricky for the decision-makers to "see" the bigger picture. Like the soldiers in the film, they had to do their best to anticipate threats and neutralize them—without a ton of knowledge or know-how.
They neutralized—or attempted to neutralize—those threats by deploying a massive armed force in South Vietnam in the mid 60s. The ensuing conflict, which only escalated as the 60s wore on, became known as the Vietnam War, and it is this "war" that is the larger setting of Platoon. We say "war" because technically the whole Vietnam thing wasn't technically a war. Congress never actually formally declared war, so the whole thing was, strictly speaking, a "conflict" or "police action."
Whatever the case, Vietnam changed just about everything.
Unlike previous wars, in which there was a clearly defined front line, and clearly defined territorial goals, Vietnam was a war of attrition, a race to see who could kill the most enemy troops or shed the most enemy blood.
And a bloody, bloody mess it was.
U.S. troops spent most of their time trekking through the jungle, fighting an enemy that was safely ensconced in elaborate bunker complexes (like the one the platoon finds about halfway through the film) and that, for all intents and purposes, had the upper hand: better knowledge of the terrain, the ability to tap into a large network of rural sympathizers (like the village the platoon torches), and a more powerful motivation to fight.
The difficult terrain, the new objectives of warfare (bodies, not territory), and a whole host of other problems made Vietnam a bloody mess. The fact that Vietnam was the first war to feature a substantial media presence (Source) meant that all the folks stateside were better informed about the horrors of war, and thus better equipped to protest the ridiculousness of the whole thing.
Calling All Hippies
The fact that Vietnam took place during the social and cultural upheaval of the 60's (Source) didn't help matters, either. Many of those issues (especially race relations) found their way to Vietnam, which hinted at in the film in the fact that the African American characters (Junior, King, Big Harold) tend to stick together, and in Junior's frequent comments about race relations ("y'all been trying to keep the black man down, and string him out on that s*** [marijuana]").
The horrors of war, coupled with the seeming purposelessness of Vietnam, ramped up tensions among the troops themselves as well and made lots of guys do lots of things they might not otherwise have done. Incompetent leadership and frustration among troops made fragging a serious concern. Elias' and Barnes' death are both examples of this as was the very real fear that seemingly innocuous farmers would turn out to be communist sympathizers led to countless atrocities.
So yeah, when you think of Platoon's setting, if you think "all around nightmare," you're pretty much on the right track.