There is no question mark at the end of my title. I ask you to grant that something is wrong with colonialism. That in itself is no meagre concession; the history of liberalism is replete with apologies for colonialism as well as condemnations of it. If you think that colonialism is justified, you are unlikely to find my arguments attractive. But if you agree that something must be wrong with colonialism, you might want to know more about what exactly the nature of the wrong is. It is tempting to answer the question by following one of two prominent strategies for showing the wrong of colonialism: an argument from nationalism and an argument from territorial rights. This article defends an alternative account. It argues that the wrong of colonialism consists in the creation and upholding of a political association that denies its members equal and reciprocal terms of cooperation.