I have no idea what Karl Marx thought, but Milton Friedman and JM Keynes, the leading capitalist economists of the last 100 years, were both fans of free trade. And actual Marxists are staunch opponents of free trade, ranging from the Warsaw Pact countries of the Cold War era to any number of African kleptocracies. In fact, the poorest countries in the world tend to have the biggest restraints on foreign trade.
Bull crap.
But, in general, the protective system of our day is conservative, while the free trade system is destructive. It breaks up old nationalities and pushes the antagonism of the proletariat and the bourgeoisie to the extreme point. In a word, the free trade system hastens the social revolution. It is in this revolutionary sense alone, gentlemen, that I vote in favor of free trade.
-- Karl Marx 1848