The western side of the Rio Grande presents a very different case from that of Texas, settled by our own people, and which but a few years ago was little better than a wilderness, a frontier to Mexico, as well as to us. The States of New Leon, Coawilla, and Tamaulipas, contain half a million of people, have been settled two hundred years, and contain ancient cities and towns. They are spread over a surface as large as Virginia and the Carolinas, and backed by other more extensive States towards Mexico. If the struggle lay only between the Mexican military and our armies, a few decisive battles might end the contest. But we have to overcome the prejudices, ignorance, and antipathies of the population, a conquest a thousand times more difficult than that of arms. And are we certain that the people of those States will remain perfectly passive, and that their countrymen beyond the Sierra Madre, cannot be rendered formidable as guerrillas? Our estimate of them may be too low. In case of some severe reverse, their numbers, should they rise _en masse_, may overwhelm detached bodies of our troops. The proclamation of Ampudia, denouncing as traitors, all who will hold intercourse with our people, has had its effect. The people will become exasperated at the outrages which will, in all probability, be committed by our irregular troops, and the guerrilla warfare will bristle over the whole country. Few among us are aware of its vast extent. It is for the greater part composed of barren mountains and arid plains, interspersed with fertile valleys, and entirely unlike our western States. We will have to guard a frontier of two thousand miles, from Santa Fee to Matamoras, without mentioning California. We will have to garrison all the principal towns between the Rio Grande and the Sierra Madre. If we attempt to advance beyond Monterey, we must force our way through a population of several millions, after crossing a desert of several hundred miles. Conquests are easy enough, when people are willing to be conquered, but when they determine to resist, it is a very different matter. When Napoleon attempted the conquest of Spain, after the manner that Edward I undertook that of Scotland, he had possession of Madrid and all the principal cities, with five hundred thousand of the best troops in the world, and yet, in less than three years, his Generals were driven out with a remnant of thirty thousand men! The capture of Burgoyne and of Cornwallis, show what a critical thing it is for an invading army to penetrate an enemy’s country, with the wave of an unconquered people closing behind them. Our armies are about to operate in a country without roads, without supplies or resources, through defiles, over deserts without water, and under a burning sun. I have great