Thus war may arise between two parties in a single State, and when it does the two parties become, to all intents and purposes, separate, independent, and sovereign States for the time being, and are, for the most part, so regarded and treated by other independent States not taking part in the conflict. For this reason, though the origin of a civil war may differ widely in all its circumstances and conditions from that of a war between two separate States, sovereign and independent _ab initio_, yet as soon as a state of war is established, as distinct from that of a puny revolt or a petty rebellion, there is, for a student of war, no practical difference between a civil war and any other kind of war. Both fall under the definition of war as the armed conflict of national wills. Between two separate, sovereign, independent nations a state of war arises in this wise. We have seen that the internal policy of an independent State is subject to no direct external control. But States do not exist in isolation. Their citizens trade with the citizens of other States, seeking to exchange the products of their respective industries to the advantage of both. As they grow in prosperity, wealth, and population, their capital seeks employment in other lands, and their surplus population seeks an outlet in such regions of the earth as are open to their occupation. Thus arise external relations between one State and another, and the interests affected by these relations are often found--and perhaps still more often believed--by one State to be at variance with those of another. In pursuit of these interests--which, as they grow and expand, become embodied in great consolidated kingdoms, great colonial empires, or great imperial dependencies, and tend to be regarded in time as paramount to all other national interests--each State formulates and pursues an external policy of its own which may or may not be capable of amicable adjustment to the policy of other States engaged in similar enterprises. It is the function of diplomacy to effect adjustments such as these where it can. It succeeds much more often than it fails. Conflicting policies are deflected by mutual agreement and concession so as to avoid the risk of collision, and each State, without abandoning its policy, modifies it and adjusts it to the exigencies of the occasion. Sometimes, however, diplomacy fails, either because the conflicting policies are really irreconcilable, or because passion, prejudice, national ambition, or international misunderstanding induces the citizens of both States and their rulers so to regard them. In that case, if neither State is prepared so to deflect its policy as to avert collision, war ensues. The policy remains unchanged, but the means of further pursuing it, otherwise than by an appeal to force, are exhausted. War is thus, according to the famous definition of Clausewitz, the pursuit of national policy by other means than those