The United States, bordered by two oceans, is a maritime nation. Not only is its Navy the largest in the world by far, but its coast guard would qualify as the 12th largest fleet in the world. The U.S. Navy is America's foremost strategic instrument -- much more so than its nuclear arsenal, which in all probability can never be used. The U.S. Navy is on the high seas around the world in peacetime as well as in wartime, guarding the sea lines of communication and the main maritime choke points. This, in turn, allows for a free global trading regime and guarantees access to hydrocarbons for America's allies. This Navy, by the way, also allows for an inland strike capacity. To wit, America bombed Iraq, Afghanistan, and Kosovo from warships in the Indian Ocean and the Adriatic Sea.
Historically, America is not unique in this regard. Athens, Venice, and Great Britain were all great global maritime powers. So were Holland and Portugal. Maritime powers, with exceptions of course, were generally more benign than land powers such as Germany and Russia. For while armies invade, ships make port visits and facilitate commerce. Navies also do not occupy foreign territory to anywhere near the extent that land forces do. While armies are required for unpredictable contingencies, navies (and air forces) project power on a daily basis. America might have gotten into unnecessary wars in Vietnam and Iraq, but American power is undiminished, largely because of the size of its Navy and Air Force. Finally, the U.S. Navy helps keep America engaged but out of trouble.
If we consider ourselves a maritime nation, chances are that we will make fewer mistakes in foreign policy, since naval power is about protecting commerce and a free trading order more than about having imperial-like possessions and interests. This is why a Navy can deploy anywhere all the time, though sending large numbers of ground troops overseas often involves a debate in Congress.
The United States currently has a Navy with almost 300 warships. This is an important fact, since if America's Navy had only, say, 200 warships, the world would be a very different place. It would be considerably more violent and anarchic than it already is. The U.S. 7th Fleet essentially keeps the peace in East Asia, while the U.S. 5th Fleet helps prevent war between Iran and the Arabian Gulf states, including Saudi Arabia.
This 300-ship Navy, combined with America's other armed services, gives the United States more power than any other nation in the world. Yet we must keep in mind that this considerable power does not equal overwhelming power. It is in the space between these two concepts that much of the violence and instability in the world takes place. For even a 600-ship Navy, or even an army much larger than our present one, would be unable to prevent the collapse of states across the Middle East. America, in other words, while guarding its interests, must be prepared to tolerate a world where it is not in control.
Let me elaborate on this.
America's ability to influence the world will likely decrease, but the ability of other powers to do likewise will also decrease over time, owing to internal economic challenges in China, Russia, and Europe that dwarf America's own economic problems. Thus our power will increase relative to other major states and unions, even as it decreases in absolute terms around the world. In all this, our Navy will be a barometer for our national health: this is because maritime platforms are frightfully expensive, thus the ability to maintain a Navy the size of ours requires public support through taxes and a healthy rate of increase in the gross domestic product.
A big Navy says a lot about who we are. And given the structural economic weaknesses of China, Russia, and Europe, I believe it is questionable whether they can keep up with the U.S. Navy over the long run. The big unknown is not Russia, which is a near-term threat rather than a long-term one, but China: Can it really reform its economy? I am not sure.
So watch the size and health of the U. S. Navy. It is as sure an indicator as any of American geopolitical power.