It is, of course, idle to press the doctrine of the command of the sea and the principle of the fleet in being so far as to affirm that no military enterprise of any kind can be prosecuted across the sea unless an unassailable command of the sea has first been established. Such a proposition is disallowed by the whole course of naval history, which is, in truth, for the most part, the history of the command of the sea remaining in dispute, often for long periods, between two belligerents, the balance inclining sometimes to one side and sometimes to the other, according to the fortune of war. The whole question is in the main one of degree and of circumstances. Broadly speaking, it may be said that the larger the military enterprise contemplated the more complete must be the command of the sea before it can be prosecuted with success and the more certain the assurance of its continuance in unimpaired efficiency until the objects of the enterprise are accomplished. Conversely, the strength, even if inferior, of the fleet in being, its strategic disposition, its tactical efficiency, and, above all, its _animus pugnandi_ must all be accurately gauged by a naval commander before he can safely decide that a military expedition of any magnitude can be undertaken without fear of interference from an enemy's fleet. It was the neglect of these principles that ruined the Athenian expedition to Syracuse. It was equally the neglect of the same principles that entailed the failure of Napoleon's expedition to Egypt and the ultimate surrender of the army he had deserted there. It was the politic recognition of them that, as Admiral Mahan has shown in a brilliant passage, compelled Hannibal to undertake the arduous passage of the Alps for the purpose of invading Italy instead of transporting his troops by sea.