The author is aware that actual invasion of a country from the sea would be a greater disaster than blockade, and that defense against invasion has often been urged in Great Britain as a reason for a great navy; so that the primary reason for a navy might be said to be defense against invasion. But why should an enemy take the trouble to invade us? Blockade is easier and cheaper, and can accomplish almost everything that an enemy desires, especially if it be enlivened by the occasional dropping of thousand-pound shells into Wall Street and the navy-yard. While, however, the _primary_ use of naval power seems to be to prevent blockade, a navy, like any other weapon, may be put to any other uses which circumstances indicate. For instance, the Northerners in the Civil War used the navy not to prevent blockade, but to make blockade; the Japanese used the navy to cover the transportation of their armies to Manchuria and Corea; and Great Britain has always used her navy to protect her trade routes. A general statement of the various uses of a navy has been put into the phrase "command of the sea." Of course, the probability of getting "command of the sea," or of desiring to get it is dependent on the existence of a state of war, and there are some who believe that the probability of our becoming involved in a war with a great naval nation is too slight to warrant the expense of money and labor needed to prepare the necessary naval power. So it may be well to consider what is the degree of probability. This degree of probability cannot be determined as accurately as the probabilities of fire, death, or other things against which insurance companies insure us; for the reason that wars have been much less frequent than fires, deaths, etc., while the causes that make and prevent them are much more numerous and obscure. It seems clear, however, that, as between two countries of equal wealth, the probability of war varies with the disparity between their navies, and unless other nations are involved, is practically zero, when their navies are equal in power; and that, other factors being equal, the _greatest probability of war is between two countries, of which one is the more wealthy and the other the more powerful_. In reckoning the probability of war, we must realize that _the most pregnant cause of war is the combination of conflicting interests with disparity in power_. And we must also realize that it is not enough to consider the situation as it is now: that it is necessary to look at least ten years ahead, because it would take the United States that length of time to prepare a navy powerful enough to fight our possible foes with reasonable assurance of success.