… is from pages 127 of Alexander Cairncross’s 1976 essay “The Market and the State,” which is chapter 4 in the 1976 collection The Market and the State: Essays in Honour of Adam Smith (Thomas Wilson and Andrew S. Skinner, eds.):
The forces of decentralization are reinforced by the international character of the market as opposed to the national character of the state. So long as the domestic market is open to competitive influences from outside, a limit is set to monopoly powers of all kinds: whether exercised by groups of producers or groups of workers, whole industries or individual firms. It was this aspect of international trade that particularly appealed to Adam Smith.