Mencius - Book 1, Part 2, Chapter 10

The people of Ch'î attacked Yen, and conquered it.

The king Hsüan asked, saying, 'Some tell me not to take possession of it for myself, and some tell me to take possession of it. For a kingdom of ten thousand chariots, attacking another of ten thousand chariots, to complete the conquest of it in fifty days, is an achievement beyond mere human strength. If I do not take possession of it, calamities from Heaven will surely come upon me. What do you say to my taking possession of it?'

Mencius replied, 'If the people of Yen will be pleased with your taking possession of it, then do so. Among the ancients there was one who acted on this principle, namely king Wû. If the people of Yen will not be pleased with your taking possession of it, then do not do so. Among the ancients there was one who acted on this principle, namely king Wan.

'When, with all the strength of your country of ten thousand chariots, you attacked another country of ten thousand chariots, and the people brought baskets of rice and vessels of congee, to meet your Majesty's host, was there any other reason for this but that they hoped to escape out of fire and water ? If you make the water more deep and the fire more fierce, they will in like manner make another revolution.'
~ James Legge translation via nothingistic.org ~

Go here to read the introductory post to this serialized version of the Works of Mencius.