With North America secured, the only other immediate threat came from Latin America. In reality, North and South America are islands, not really connected: Panama and Central America are impassable by large armies. South America's unification into a single entity is remote. When you look at a map of South America, leaving out impassable terrain, you see that there can be no transcontinental power: the continent is sliced in two (see map, page 43). So there is no chance of a native threat to the United States emerging from South America.
The major threats in the hemisphere came from European powers with naval bases in South and Central America and the Caribbean, as well as land forces in Mexico. That is what the Monroe Doctrine was about — long before the United States had the ability to stop the Europeans from having bases there, it made blocking the Europeans a strategic imperative. The only time the United States really worries about Latin America is when a foreign power has bases there.
In 1812, the British navy sailed up the Chesapeake and burned Washington. Throughout the nineteenth century, the United States was terrified that the British, using their overwhelming control of the North Atlantic, would shut off its access to the ocean, strangling the United States. It was not always a paranoid fear: the British did consider this on more than one occasion. This general problem was, in other contexts, the origin of the American obsession with Cuba, from the Spanish-American War through the Cold War.
Having secured the hemisphere in the late nineteenth century, the United States has an interest in keeping the sea lanes approaching its borders free of foreign naval power. The United States secured its Pacific approaches first. During the Civil War it acquired Alaska. In 1898 it annexed Hawaii. Those two actions taken together closed off the threat of any enemy fleet being able to approach the continent from the west, by eliminating any anchorage for supplying a fleet. The United States secured the Atlantic by using World War II to take advantage of British weakness, driving it from near the U.S. coast, and by the end of World War II had created a fleet of such enormous power that the British were unable to operate in the Atlantic without U.S. approval. This made the United States effectively invulnerable to invasion.
The fact that the United States emerged from World War II not only with the world's largest navy but also with naval bases scattered around the world changed the way the world worked. As I mentioned previously, any seagoing vessel — commercial or military, from the Persian Gulf to the South China Sea to the Caribbean — could be monitored by the United States Navy, who could choose to watch it, stop it, or sink it. From the end of World War II onward, the combined weight of all of the world's existing fleets was insignificant compared to American naval power.
