At times tribes joined in alliance with other tribes to attain a common end, the alliance being brought about by peaceful agreement, and usually ceasing after the end had been attained, or missed, or when tribal jealousies forbade further common effort. Sometimes tribes joined to form one larger tribe; the union being either forced on a weaker by a stronger tribe, or caused by a desire to secure a strength greater and more lasting than mere alliance can insure. In the same way, and apparently according to similar laws, sovereign states or nations were formed from tribes; and in later years, by the union of separate states. The states or nations have become larger and larger as time has gone on; greater numbers, not only of people but of peoples, living in the same general localities and having hereditary ties, joining to form a nation. Though the forms of government of these states or nations are numerous, and though the conceptions of people as to the purposes and functions of the state vary greatly, we find that one characteristic of a state has always prevailed among all the states and nations of the world--the existence of an armed military force, placed under the control of its government; the purpose of this armed force being to enable the government not only to carry on its administration of internal matters, but also to exert itself externally against the armed force of another state. This armed force has been a prominent factor in the life of every sovereign state and independent tribe, from history's beginning, and is no less a factor now. No instance can be found of a sovereign state without its appropriate armed force, to guard its sovereignty, and preserve that freedom from external control, without which freedom it ceases to exist as a sovereign state. The armed force has always been a matter of very great expense. It has always required the anxious care of the government and the people. The men comprising it have always been subjected to restraint and discipline, compelled to undergo hardships and dangers greater than those of civil life, and developed by a training highly specialized and exacting. The armed force in every state has had not only continuous existence always, but continuous, potential readiness, if not continuous employment; and the greatest changes in the mutual relations of nations have been brought about by the victory of the armed force of one state over the armed force of another state. This does not mean that the fundamental causes of the changes have been physical, for they have been psychological, and have been so profound and so complex as to defy analysis; but it does mean that the actual and immediate instrument producing the changes has been physical force; that physical force and physical courage acting in conjunction, of which conjunction war is the ultimate expression, have always been the most potent instruments in the dealings of nations with each other.