Regardless of whether each of the many settings in the book is a war zone, each is harsh and hostile, with its own particular set of challenges. The challenges of Vietnam are obvious; it is a war zone. There is nobody who can be trusted, and there is a constant threat of death. Beyond that, the terrain is almost impossible and there are bugs, filth, and humidity. The same is true of the Philippines where the conditions are unlivable.
This feeling that one is always living somewhere hostile is what creates the feeling in the novel of constantly being under threat, whether in the war zone or not. Everything is alien and foreign. There is nobody who can truly be trusted, no terrain that can be relied upon and no situation that turns out as expected, even when it is clearly strategized. This creates a feeling of perpetual fear. None of the characters can relax and paranoia begins to exhibit in a number of ways, to varying degrees.
The Colonel, having been a rational man for the duration of his military career, begins to see the enemy around every corner. In today's world he would be a conspiracy theorist; in the novel, even his own nephew seems to be pitted against him, from the Colonel's own perspective, and he no longer believes that he can trust anyone's judgement but his own. This is ironic, though, because his own judgement is failing and he is falling victim to his own paranoia every day.
The change in the Colonel also precipitates the orchestrated downfall of Skip Sands. He realizes too late that the role he had tried to carve out for himself had already been prepared for him. He was chosen to be the fall guy. The landscape around him is filled with potholes and obstacles that are designed to make it appear that he has failed, because a failure by the Colonel is not something that is going to be allowed to see the light of day. Skip finds his surroundings more and more alien so that eventually even his own countrymen seem like foreigners in whom he has nothing in common.
The main tenet of the book - camaraderie in war - suffers as a result of this alien terrain, until by the end of the novel, the characters do not even trust, or recognize, themselves.
