Edit - Since John so kindly pointed out that this quote doesn't make much sense without any context, I'll now provide some. The first I'd heard this quote referenced was by military historian Martin Van Creveld who used the metaphor to describe what happens to military forces which find themselves fighting a nonconventional insurgency (France in Algeria, Soviet Union in Afghanistan, United States in Vietnam, etc.) Van Creveld was one of the first military scholars to recognize and identify that interstate warfare would be replaced by small scale intrastate conflicts and that the role of the military would shift towards global policework, and that the military was thus trained for the wrong kind of war.
According to Van Creveld, it is very difficult (nigh impossible) for a strong nation to occupy a weaker nation and fight an insurgency for any extended period of time and win.
In private life, an adult who keeps beating down on a five year old – even such a one as originally attacked him with a knife – will be perceived as committing a crime; therefore he will lose the support of bystanders and end up by being arrested, tried and convicted. In international life, an armed force that keeps beating down on a weaker opponent will be seen as committing a series of crimes; therefore it will end up by losing the support of its allies, its own people, and its own troops. Depending on the quality of the forces – whether they are draftees or professionals, the effectiveness of the propaganda machine, the nature of the political process, and so on – things may happen quickly or take a long time to mature. However, the outcome is always the same. He (or she) who does not understand this does not understand anything about war; or, indeed, human nature.