I am looking for identification of a short story I read as part of an anthology around twenty years ago, and I think it might have been written by Harry Harrison but I am not certain.
Plot line goes like this: evil alien empire invade earth. They've done this many times, they know the routine. The populace resists, is crushed and turned into slaves/subjects of the empire, leader of the invasion retires to a life of luxury as colonial governor. I think the aliens look like humanoid snakes.
But Earth is different. When the invading force arrives, they spot no defenses, no military. When the ships land, the earthlings welcome them with open arms. The aliens are nonplussed and ask if the earthlings realize just what is going on.
Oh yes, the earthlings reply. But they point out that Earth doesn't have FTL technology and clearly the aliens do and could crush them if they resist. But by the same token they must have value, and other out there (whether pirates or other empires) may seek to take their planet by force and kill the populace. And if that happened, the empire would be forced to defend Earth as a valuable colony. Therefore it is in the Earthlings' best interests to be as valuable to their new masters as possible, not fight them.
The aliens point out that they could conceivably be defeated by this hypothetical threat. The humans shrug and agree, then point out that this would simply show that their new masters were even more capable of defending Earth.
Fast forward and the new planetary governor finds himself bored. He is chatting with his second in command who has just come in excitedly for having earned a black belt in human martial arts (I don't remember which one in particular, if it is even mentioned).
The governor is mystified, why didn't his lieutenant simply take this black belt? The lieutenant explains that without earning the belt, it would be meaningless. Instead, it required him to train and master the skill of human martial arts, which are curiously different from the aliens' own form. He explains via example:
In the alien martial arts, if one person shoves against the other, the second person seeks to shove back harder, thereby establishing might and dominance.
In human martial arts, however, the second person would instead seek to grab the first persons arm and guide them in the direction they are already going. By doing so, the first person is thrown completely off balance and at the mercy of the second person.
The governor thinks this sounds insane, ridiculous, and says a lot about a cowardly people like the humans.
Toward the end of the story, this comes back to haunt him. He suddenly finds that none of his men will respond to his command. A coup has occurred and the humans have slaughtered every alien on the planet except him. The humans simply bided their time until they had learned the secret of FTL, how to build and maintain a space navy, and in the meantime the occupying force had become soft and were easily dispatched. Now the humans are taking the navy back to the alien empire and aim to take it down. To replace it or to free the enslaved planets, I don't really remember.