I've heard this axiom split up into two parts.
Rules of Conquest
- Rape before you pillage.
- Pillage before you burn.
At first this seems like common sense, but when you think about it, it's good advice for any budding conqueror. Failure to rape first allows potential victims time to run away. This is bad. A lack of conquered native people isn't terribly conducive to having anyone to rape. Sure there may be some stragglers remaining behind, but all the choice genes will have already gotten the heck out of Dodge.
Pillaging before you burn is also a good idea. Premature burning will lead to a lack of decent booty. In fact you could take that two ways. Please feel free to do so. So burn last. Or perhaps you can skip the burning if you're feeling lazy. There's no need to raze the city if your goal is to live there. Unless you disagree with their taste in architecture. However if you're going to move on to conquer other cities then burning is a good thing. You don't want to let the natives rebuild too fast. They might get an army and come back to bite you in the arse.
Standard Before Anyone Gets Their Panties In A Bunch Disclaimer: The preceding was a joke. Sarcasm. Not serious. So if you've taken issue with it, then get over yourself. Please.