The work that best examines Keynes’s opinions upon the free trade issue are written by him in "National Self-Sufficiency" The Yale Review, Vol. 22, no. 4 (June 1933).
“Like most Englishmen” writes Keynes, he in his youth “respect[ed] free trade not only as an economic doctrine which a rational and instructed person could not doubt, but almost as a part of the moral law.” However, these views would change as the result of the First World War and the economic crises that followed in the 1930s. Keynes describes free trade as “the mental habits of the pre-war nineteenth century world.”, it was an ideology that he saw as obsolete to solve the economic problems of a world exiting World War One. Accusing free trade proponents of being “among the most idealistic and disinterested of men” Keynes proceeds to critique free trade as being advertised as a tool for world peace, by making the nations of the world codependent upon each other. Keynes wrote that these assumptions ignored the problems of nationalism and geopolitics, using the First World War as an example on how countries involved together in free trade could turn violent against each other. “…the age of economic internationalism was not particularly successful in avoiding war; and if its friends retort, that the imperfection of its success never gave it a fair chance, it is reasonable to point out that a greater success is scarcely probable in the coming years.”
“Ideas, knowledge, science, hospitality, travel--these are the things which should of their nature be international. But let goods be homespun whenever it is reasonably and conveniently possible, and, above all, let finance be primarily national.”
Despite his advocacy for self-sufficiency, Keynes saw three “outstanding dangers” in economic nationalism. The first of these was “silliness of the doctrinaire”, that leaders pushing for it abused poetic license when detailing its virtues and vices. Rhetoric, especially that of those in authority and power, would have to remain mature for it to be efficient.
The second was haste. As Paul Valery wrote: “Political conflicts distort and disturb the people's sense of distinction between matters of importance and matters of urgency.” Using the example of communist Russia, Keynes argued that a swift political revolution would lead to unnecessary sacrifice and loss, thus discrediting the movement.
The third was intolerance and the ignoration of criticism. Said Keynes, the new nationalist movements did not convince their opponents, they toppled them.
The essay can be read in its entirety here: https://www.mtholyoke.edu/acad/intrel/interwar/keynes.htm
In 1941, Keynes proposed a system in which there would be a global bank, the International Clearing Union, which issued its own currency, the bancor. Bancors would be interchangeable with national currencies at fixed rates of exchange, thus they could be used to easily measure trading deficits and surpluses. The bank would charge interest upon a country with a deficit, making the country’s currency reduced and preventing capital export. As well countries with surpluses would be charged with interest to increase the value of their currency and permit capital export; this, per Keynes, insured there would be no trading deficits or surpluses at any time. This was Keynes’s pet project, a model for a world economy without free trade, and it was officially endorsed by Britain for the 1944 Bretton Woods Conference, a conference in which all 44 allied nations discussed plans for the future world economy. It was rejected in favor of Harry Dexter White’s International Stabilization Fund, the blueprint for what would later become the International Monetary Fund. To improve the economies of devolving nations, the Conference as well pasted the idea for White’s International Bank for Reconstruction and Development. Keynes was defeated, and free trade had won as the model for the international post-war economy.