The dominant geographical feature of the landmass comprising the United States is the Mississippi River basin, an area that has more kilometers of navigable internal waterways than the rest of the world combined. The many natural and easily navigable waterways here greatly reduce the costs of transportation from the agricultural regions in the American Midwest — which is itself the world’s largest contiguous piece of farmland — to not just the rest of the country but the world.

Stretching into Montana and Minnesota in the far north and the Appalachian Mountains in the east that separate the historical core of the United States from the rest of North America, the Mississippi and its tributaries act as an economic and political unifier of much of the continent. Whereas the states of New England and the Old South, which were both settled overwhelmingly by Englishmen, developed unique cultures of their own, with many particularities carried over from Europe, the areas north and west of Appalachia, which were settled mostly by Northern European immigrants, formed a cultural whole.

Historian Frederick Jackson Turner famously argued in his Frontier Thesis that the sheer presence of abundant and uncultivated arable land west of the Appalachians combined with what America’s 26th president, Theodore Roosevelt, described as “the winning of the west” to forge a new and rugged identity, one that spread into the “Wild West” and Pacific Ocean states and came to dominate the American mindset.

The completion of the first transcontinental railway in 1869 completed the conquest of the west and unified the North American continent under one nation’s leadership. While the areas west of the Rocky Mountains are not critical to American security, they are of substantial economic and strategic value. They make America a Pacific power while California’s Central Valley is the primary source for a number of food products throughout the United States. It provided 8 percent of the nation’s agricultural output by value in 2012.

Largely insulated from the rest of the world by two oceans and separated by lakes and forests from Canada’s population centers, the United States’ weak point in the nineteenth century lay to the south. In particular New Orleans, which commands the entire maritime network upon which the nation depends, was vulnerable to attack. Hence the 1845 incorporation of Texas and a war with Mexico the following year that pushed the border into the Chihuahuan Desert.

To ensure New Orleans’ access to Atlantic Ocean, Florida and Cuba were taken from Spain in 1819 and 1898, respectively. The island remained a de facto American territory until the 1959 revolution. When it subsequently aligned with the Soviet Union, the United States went to the brink of nuclear war to prevent it from menacing the mainland.

Spain also ceded Guam, the Philippines and Puerto Rico to the United States in 1898. The Panama Canal Zone was put under American control in 1904. This put the whole “American Mediterranean” under the country’s sway, enabling it to project power into South America north of the Amazon rainforest. Further south, it faced no threats. The topography of South America prevents the emergence of a rival power. The areas east of the Andes Mountains are cut off from the rest of the continent and have no rivers to facilitate north-south integration. Argentina is endowed with a favorable climate and geography, but too far removed from the rest of the world to play a role internationally.

America, then, is the natural hegemon of the Western Hemisphere and once it had consolidated its position north of the Amazons, it had “power to spare for activities outside the New World,” wrote the geostrategist Nicholas J. Spykman in America’s Strategy in World Politics: The United States and the Balance of Power (1942).

At no point in history was that more obvious than immediately after World War II. America’s gross national product was half the entire world’s at that time. The decline of the British Empire, from which the United States had emerged two centuries earlier, allowed it to take over naval stations in Caribbean. It established the North Atlantic Treaty Organization in 1949 which gave it port access across the Atlantic and Mediterranean Sea; occupied Japan and turned it into a protectorate; and allied with Australia and New Zealand, extending American naval hegemony into the South Pacific by 1951.

With America thus supreme on the world’s oceans, only the landmass of Northern Eurasia can produce a rival. Between the North European Plain (which is under America’s control), the Eurasian steppe (Russia) and the Yellow River basin (China), it holds even more arable land than North America but the three are not connected by waterways. Keeping these regions divided and unable to challenge the United States either in unison or on their own must be the country’s overarching strategic priority.