In book 4 chapter 2 of “The Wealth of Nations”, Smith was arguing that foreign trade is the least beneficial form of economic activity. Smith's (but not Samuelson’s) invisible hand directed individuals to favour the domestic rather than the international economy, even on occasions when the profits to be gained from international trade were higher.
Smith opposed trade protection, not because he favoured international laissez-faire, but because he believed the invisible guiding hand would render protection of domestic economies unnecessary. He believed that humans did not need the means of protection in order to achieve the goals of protectionism. His central ethic was that of patriotism.