From a free trade petition put out by the the Atlas Global Initiative for Free Trade, Peace, and Prosperity:
But the fact that protectionism destroys wealth is not its worst consequence. Protectionism destroys peace. That is justification enough for all people of good will, all friends of civilization, to speak out loudly and forcefully against economic nationalism, an ideology of conflict, based on ignorance and carried into practice by protectionism.
Two hundred and fifty years ago, Montesquieu observed that “Peace is the natural effect of trade. Two nations who differ with each other become reciprocally dependent; for if one has an interest in buying, the other has an interest in selling; and thus their union is founded on their mutual necessities.”
Trade’s most valuable product is peace. Trade promotes peace, in part, by uniting different peoples in a common culture of commerce – a daily process of learning others’ languages, social norms, laws, expectations, wants, and talents.
ADDED: More on the same point at vox.