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 Forever War is only one of a large number of books in which the alien "enemy" is unknown and/or unknowable for the majority of the story, or at least to the majority of the protagonists. In the book The Killing Star (required reading) the aliens who wipe out humanity (using the very realistic tactic of relativistic bombs) remain utterly unknown and unknowable to all but two of the protagonists for the entirety of the book. When the aliens are finally revealed it only creates more questions, and to some extent the aliens are not revealed, so much as their unknowable nature is revealed even more clearly.
On any occasion where two systems come into conflict (conflict in the sense of the systems placing friction on each other) the flow is from a state of unknowing to knowing. This is not a steady progression, for reasons which will become clear. Knowledge of an opposing system can never be perfect, as there are no utterly predictable (rational) actors in a dynamic system, however over time trends will begin to emerge, an outline of the opponent and their motivations will follow.
This is where the power of the science fiction metaphor is particularly powerful. Human opponents cannot be unknown, since human entities can examine each other in the context of their mutual humanity and begin to extrapolate from that single baseline. Of course that can be misleading, since human behaviour is subject to factors which may be outside the realm of familiarity to the observer. Sociopathic behaviour, for example, can be almost knowable to a non-sociopathic observer. Alien opponents exist on the extreme far end of the spectrum of behaviour, since their objectives may be so beyond human comprehension as to not only be unknown but to achieve true unknowability (a word which almost certainly doesn't exist but for which I make absolutely no apologies).
Of course in any conflict there is a benefit in being able to retain from ones opponent true knowledge, since if one side knows, with absolute certainty all aspects of their opponent then they can defeat that opponent with minimal effort. This again is encapsulated in stories such as The Killing Star, where the first attack is also the one which wipes out all but the last vestiges of the human race. Due to a slight miscalculation however the aliens fail to eliminate all humanity, in essence, the human race were not entirely "known" and thus the alien strategy failed to achieve it's goal entirely. If the aliens had not missed one small but critical piece of data (full knowledge of human developmental rates) then they would have been able to attack with complete effectiveness.
To be truly effective any activity of significance needs to be underpinned by a strategic framework. This is particularly true in any environment in which hostility (friction, resistance) is likely to be encountered, in essence this covers the vast majority of activity. To form a strategy something must be known about the friction which is likely to be encountered (particularly it's overarching nature, is it human, environmental, natural, artificial etc).
A metaphor for this would be a military unit, advancing across regular terrain (low friction) and then encountering an area where the terrain has been turned over and broken up. Two options exist, one, this state is natural (perhaps animals have dug it up while foraging), two, it is artificial (the opponent planted landmines and was sloppy about covering their activity). The tactical approach to the problem of how to deal with this new terrain will be guided by the strategy based on an understanding of the environment, encompassing both the opponent and the landscape. If zero knowledge is available (a state of unknowing) it is likely that the unit will advance through the manipulated terrain and in so doing so will increase knowledge (possibly to their personal detriment).
The less that is known the worse that the strategy will be, and the lower it's effectiveness, with the opposite being evident. The most severe problem in any human/human conflict (outside of war) tends to be biases, particularly confirmation biases, which kick in as soon as observed reality starts to differ from the mental models of the observer. This is a wilful state of unknowing, deliberately placing oneself in a position where reality is rejected.
Fundamentally one must achieve a state in which all actions not only progress the existing strategy, and the reaction of the opponent (and all sources of friction) is registered and recorded, creating a virtuous feedback loop. Thus knowledge of the opponent increases steadily. At the same time one's own actions must remain mysterious where possible, to ensure that you remain as unknown (and ideally unknowable) to your opponent.