The Forever War is, unsurprisingly, a book about war. It charts the conflict between humanity and an alien species called Taurians. The Taurians are alien in the classic sense, unknown and for the majority of the book unknowable.
The core of the book is the war, but it is also a book about disconnection, since the protagonists suffer from the effects of time dilation every time they go on tour, with hundreds of years potentially passing each time they ship out and return home. The same is true of the Taurians, meaning that in every engagement is is impossible to know whether the Taurian enemy is operating with technology from the future, or the past (from the perspective of the protagonists).
Over time the characters become disconnected from their own species, as guided evolution turns humanity into a species in which they have no part. Despite being the cream of the crop when recruited (all recruits have an IQ of 150+, the effects of this brain drain are explored in the book) they are left behind by a humanity which has chosen to pursue a guided evolution.
Strategy is at it's most effective when the environment over which a conflict is going to be fought is understood. Terrain is part of this, however the mindset and moral elements of the opponent must also be understood. The Forever War is a study in what happens when a conflict is unmoored from reality, indeed it never has a root in reality, since the Taurians are unknown and unknowable.
Similarly in any real world situation where strategy must be employed the greater the comprehension of the reality the stronger the strategy will be. Boyd would have called this comprehension "Observation", the Fingerspitzengefühl from which the originator of strategy is able to build. In the case of the Forever War the Fingerspitzengefühl is entirely lacking, the creators of the war know only that an alien species exists, but beyond that have no understanding. Everything which the protagonists do in the book (in the conduct of the war) is tactical, go here, take that, kill this, but they have no sense of a wider strategy, there is only the objective of killing off the Taurian race.
On a side note, there are strong lessons here about counter insurgency. Since insurgency is to a large extent a moral activity (in the eyes of it's protagonists) the only way to be absolutely certain that an insurgency is ended (rather than having receded, to emerge later) is to kill 100% of those who might be sympathetic to the insurgency. In human-human conflict the moral compunction not to kill mostly prevents this, however in hypothetical human-alien conflict no such moral compunction exists (at least not until circumstances arise in which humanity is glimpsed in the alien, at which point they cease to be alien in the true sense and instead become funny looking "people"). In the Forever War it is not until the alien aspect of the opponent eliminated that the war can be ended.
I'll end with a quote, from someone who would have instinctively understood the implications of The Forever War:
It is said that if you know your enemies and know yourself, you will not be imperiled in a hundred battles; if you do not know your enemies but do know yourself, you will win one and lose one; if you do not know your enemies nor yourself, you will be imperilled in every single battle.
- Sun Tzu