To act against an enemy who is here to-day and there to-morrow; who at one time stampedes a herd of mules upon the head waters of the Arkansas, and when next heard from is in the very heart of the populated districts of Mexico, laying waste haciendas, and carrying devastation, rapine, and murder in his steps; who is every where without being any where; who assembles at the moment of combat, and vanishes whenever fortune turns against him; who leaves his women and children far distant from the theatre of hostilities, and has neither towns or magazines to defend, nor lines of retreat to cover; who derives his commissariat from the country he operates in, and is not encumbered with baggage-wagons or pack-trains; who comes into action only when it suits his purposes, and never without the advantage of numbers or position--with such an enemy the strategic science of civilized nations loses much of its importance, and finds but rarely, and only in peculiar localities, an opportunity to be put in practice. Our little army, scattered as it has been over the vast area of our possessions, in small garrisons of one or two companies each, has seldom been in a situation to act successfully on the offensive against large numbers of these marauders, and has often been condemned to hold itself almost exclusively upon the defensive. The morale of the troops must thereby necessarily be seriously impaired, and the confidence of the savages correspondingly augmented. The system of small garrisons has a tendency to disorganize the troops in proportion as they are scattered, and renders them correspondingly inefficient. The same results have been observed by the French army in Algeria, where, in 1845, their troops were, like ours, disseminated over a vast space, and broken up into small detachments stationed in numerous intrenched posts. Upon the sudden appearance of Abd el Kader in the plain of Mitidja, they were defeated with serious losses, and were from day to day obliged to abandon these useless stations, with all the supplies they contained. A French writer, in discussing this subject, says: "We have now abandoned the fatal idea of defending Algeria by small intrenched posts. In studying the character of the war, the nature of the men who are to oppose us, and of the country in which we are to operate, we must be convinced of the danger of admitting any other system of fortification than that which is to receive our grand depots, our magazines, and to serve as places to recruit and rest our troops when exhausted by long expeditionary movements. "These fortifications should be established in the midst of the centres of action, so as to command the principal routes, and serve as pivots to expeditionary columns.