Stratfor's famous analysis of the geopolitics of the US[1] makes clear that this was due to the phenomenal and unprecedented geographical conditions the US enjoyed, not merely "a dividend of free market agriculture".
> The most distinctive and important feature of North America is the river network in the middle third of the continent...
> The network consists of six distinct river systems: the Missouri, Arkansas, Red, Ohio, Tennessee and, of course, the Mississippi. The unified nature of this system greatly enhances the region's usefulness and potential economic and political power. First, shipping goods via water is an order of magnitude cheaper than shipping them via land. The specific ratio varies greatly based on technological era and local topography, but in the petroleum age in the United States, the cost of transport via water is roughly 10 to 30 times cheaper than overland. This simple fact makes countries with robust maritime transport options extremely capital-rich when compared to countries limited to land-only options. This factor is the primary reason why the major economic powers of the past half-millennia have been Japan, Germany, France, the United Kingdom and the United States.
> Second, the watershed of the Greater Mississippi Basin largely overlays North America's arable lands. Normally, agricultural areas as large as the American Midwest are underutilized as the cost of shipping their output to more densely populated regions cuts deeply into the economics of agriculture. The Eurasian steppe is an excellent example. Even in modern times Russian and Kazakh crops occasionally rot before they can reach market. Massive artificial transport networks must be constructed and maintained in order for the land to reach its full potential. Not so in the case of the Greater Mississippi Basin. The vast bulk of the prime agricultural lands are within 200 kilometers of a stretch of navigable river. Road and rail are still used for collection, but nearly omnipresent river ports allow for the entirety of the basin's farmers to easily and cheaply ship their products to markets not just in North America but all over the world.
> Third, the river network's unity greatly eases the issue of political integration. All of the peoples of the basin are part of the same economic system, ensuring constant contact and common interests. Regional proclivities obviously still arise, but this is not Northern Europe, where a variety of separate river systems have given rise to multiple national identities.
> It is worth briefly explaining why Stratfor fixates on navigable rivers as opposed to coastlines. First, navigable rivers by definition service twice the land area of a coastline (rivers have two banks, coasts only one). Second, rivers are not subject to tidal forces, greatly easing the construction and maintenance of supporting infrastructure. Third, storm surges often accompany oceanic storms, which force the evacuation of oceanic ports. None of this eliminates the usefulness of coastal ports, but in terms of the capacity to generate capital, coastal regions are a poor second compared to lands with navigable rivers.
Rather than risk copy pasting the entire thing, I'd simply urge you to read it, it's highly interesting if you want to go beyond ideological thinking (free market vs. soviets).
[1]: https://worldview.stratfor.com/article/geopolitics-united-st...