That idea was the topic of a book a few years back (well, probably about ten years ago), did Buchanan cite anything? It wasn’t his idea.
Mussolini was looking around for alliances (the European economy was very bad for years after WWI, well before the Great Depression hit in the US), was basically given the brush-off by Britain and France, then met Hitler and was wowed. Also, the goose-stepping was something that didn’t originate in WWII, obviously (I remember seeing a snippet of an English honor guard stepping that way, from film shot between the wars), but I think the Italian troops used for the welcome parade were doing that, and Hitler really liked how it looked. :’)
Hitler’s economic approach was to trade German finished goods for raw materials (most seriously for the US, he made those arrangements with variou countries in South America), making it possible for countries with insignificant industrial bases and little foreign exchange to dig some deep holes or what have you and get autos and the works. It was a smart and effective strategy for a country which had few overseas colonies (and none of much value to it) before WWI, and basically none thereafter. The lost trade for the US was estimated by the State Dep’t at $2 billions, and that was a LOT of money back then.
I have to give Buchanan his due, he writes like a serious historian, even though most historians, no doubt, do not consider him one. So the answer is "yes," everything is referenced to source material.
In this particular case, Buchanan's first point is not really controversial. Italy was allied with France and Britain during the First World War, and that alliance broke apart after the rise of Mussolini. The fault was primarily Mussolini's -- he was an arrogant SOB intent on conquest, and he offended the sensibilities of Britain's Antony Eden.
But Buchanan's more interesting point is that, at the time, Germany was still very weak, and a united front of Italy, France and Britain could well have put an early stopper on Hitler's territorial ambitions. So, by Britain's refusing to accommodate (or appease) Mussolini, they helped drive him into the much more dangerous arms of Hitler.
And, at the same time, by refusing to make nice with the disgusting dictator Mussolini, they were eventually forced to make very very nice with the far more dangerous dictator, Stalin!
So, Buchanan's theme is that Britain's interests would have been much better served if they had been more realistic, and less idealistic.