"Empires won by conquest have always fallen either by revolt within or by defeat by a rival"

This quote by John Boyd Orr recommends that empires that are won through conquest are not sustainable in the long run. Empires that are built on the subjugation of others are naturally unstable and will ultimately collapse. This is due to either internal revolt, where individuals of the empire rise up against their oppressors, or due to beat by a competing empire. This is because empires that are built on conquest are not based on mutual respect and understanding, however rather on power and supremacy. As such, they are not able to endure the test of time and will become toppled. This quote functions as a suggestion that empires constructed on conquest are not sustainable and will eventually fall.