Aliens capable of space travel happen upon earth and plan to ravage the planet. The rest of the alien technology is relatively primitive compared to earth's. They don't have clean water, they use chamber pots, as they travel in space they use persicopes to guide them... and other incongruities. Earth turns out to be a formidable foe since we have advanced our weapons technology beyond theirs. The aliens are otherwise primitive because, having the ability to travel to other worlds, no one else in the universe has needed to work out their resource problems on their own planet... they just go to another planet and steal resources. Since humans never figured out how to travel to other worlds, we've had to make due with what we have, and have concentrated our efforts inwards (rather than towards space pillaging). The secret to space travel turns out to be a simple theory of physics which humans just haven't happened upon yet. At the end, as we've defeated the aliens, one of them realizes that they've now given us the capability to travel to other planets, and with our advanced technologies, we'll soon take over the entire universe.
This was a short story presented in a collection (Art Clarke?). Any help would be appreciated. Thank you.