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Looking downward towards the west from the shoulders of the lofty elevations which border the feet of the volcano of Popocatepetl, the spectator beholds a remarkable and perfect basin, enclosed on every side by mountains whose height varies from two hundred to ten thousand feet from its bottom. The form of this basin may be considered nearly circular, the diameter being about fifty miles. As the eye descends to the levels below, it beholds every variety of scenery. Ten extinct volcanoes rear their ancient cones and craters in the southern part of the valley, multitudes of lesser hills and elevations break the evenness of the plain, while, interspersed among its eight hundred and thirty square miles of arable land and along the shores of its six lakes of Chalco, Xochimilco, Tezcoco, San Cristoval, Xaltocan and Zumpango, stretching across the valley from north to south, are seen the white walls of ten populous cities and towns. In front of the observer, about forty miles to the west, is the capital of the Republic, while the main road thither descends rapidly from the last mountain slopes, at the Venta de Cordova, until it is lost in the plain on the margin of Lake Chalco near the Hacienda of Buena Vista. From thence to the town of Ayotla it sweeps along the plain between a moderate elevation on the north and the lake of Chalco on the south.
On the 11th of August, General Scott, after crossing the mountains, concentrated his forces in the valley. General Twiggs encamped with his division in advance, on the direct road, at Ayotla, near the northern shore of Lake Chalco; General Quitman was stationed with his troops a short distance in the rear; General Worth occupied the town of Chalco on the western shore of its lake, while General Pillow brought up the rear by an encampment near Worth.
This position of the army commanded four routes to the capital whose capture was the coveted prize. The first of these, as well as the shortest and most direct, was the main post road which reaches the city by the gate or garita of San Lazaro on the east. After passing Ayotla this road winds round the foot of an extinct volcanic hill for five miles when it approaches the sedgy shores and marshes of Lake Tezcoco on the north, thence it passes over a causeway built across an arm of Tezcoco for two miles, and, by another causeway of seven miles finally strikes the city. The road is good, level, perfectly open and comfortable for ordinary travelling, but the narrow land between the lakes of Chalco and Tezcoco, compressed still more by broken hills and rocks, admits the most perfect military defence. At the end of the first causeway over the arm of Tezcoco which we have just described, is the abrupt oblong volcanic hill styled El Peñon, four hundred and fifty feet above the level of the lake, its top accessible in the direction of Ayotla at only one point, and surrounded by water except on the west towards Mexico. It is a natural fortress; yet Santa Anna had not neglected to add to its original strength, and to seize it as the eastern key of his defences. Three lines of works were thrown up, at the base, at the brow, and on the summit of the eminence. The works at the base, completely encircling El Peñon, consisted of a ditch fifteen feet wide, four and a half feet deep, and a parapet fifteen feet thick whose slope was raised eight and a half feet above the bottom of the ditch. Ample breastworks formed the other two lines of the bristling tiara. In addition to this, the causeway across the arm of Tezcoco, immediately in front, had been cut and was defended by a battery of two guns, while the fire from all the works, mounting about sixty pieces, swept the whole length of the causeway.
The second road to the capital was by Mexicalzingo. After leaving Ayotla the highway continues along the main post road for six or seven miles and then deflects southwardly towards the village of Santa Maria, whence it pursues its way westwardly towards Istapalapan, but, just before reaching Mexicalzingo, it crosses a marsh formed by the waters of Lake Xochimilco, on a causeway nearly a mile long. This approach, dangerous as it was by its natural impediments, was also protected by extensive field works which made it almost as perilous for assault as the Peñon.
The third route lay through Tezcoco. Leaving Chalco and the Hacienda of Buena Vista, it strikes off from the main route directly north, and passing through the town of Tezcoco, it sweeps westwardly around the shores of the lake of that name until it crosses the stone dyke of San Cristoval, near the lake and town of that name; thence, by a road leading almost directly south for fifteen miles, through the sacred town of Guadalupe Hidalgo, it enters the capital. It is an agreeable route through a beautiful country, yet extremely circuitous though free from all natural or artificial obstacles, until it reaches Santiago Zacualco within two miles of Guadalupe. But at the period of Scott's invasion of the valley, General Valencia, with the troops that were afterwards convened at Contreras, was stationed at Tezcoco, either for the purpose of observation, or to induce an attack in that quarter, and thus to draw our forces into a snare on the northern route, or to fall on the rear of the American commander if he attacked El Peñon, or advanced by the way of Mexicalzingo. At Santiago Zacualco, west of the lake and on the route, formidable works were thrown up to defend the entire space between the western shore of lake Tezcoco and the mountains; while on the road to Querétaro, at the mountain pass north of Tenepantla, other defences were erected, so as to screen the country on all sides of the group of hills which lies west of the lakes of Tezcoco and San Cristoval and north of the town of Guadalupe Hidalgo.
The fourth and last advance to the city was that which turned to the south from the Hacienda of Buena Vista, and passing by the town of Chalco, led along the narrow land intervening between the shores of lake Chalco and the first steeps of the mountains forming the southern rim of the valley, until it fell at right angles, at Tlalpam or San Agustin de las Cuevas, into the main road from the city of Mexico towards the southern States of the Republic.
All these routes were boldly reconnoitred by the brave engineers accompanying the American army, and, where they could not extend their personal observations, the officers obtained from the people of the country, information upon which subsequent events proved that they were justified in relying. From the knowledge thus gained as to the route south of the lake of Chalco, they were induced to believe, although it was rough, untravelled, difficult, and narrowly hemmed in between the lake and the mountains, yet that the long and narrow defile, which was open to resistance at many points, was not sufficiently obstructed or fortified to prevent our passage. All the routes on the lower lands, it should also be remembered, were liable to increased difficulties from the deluging rains prevailing at this season on the highlands of Mexico, and which sometimes convert the highways and their borders, for many leagues, into almost impassable lagunes.
Santa Anna and his engineers had probably supposed that this southern route would not be adopted, but a reasonable explanation of his conduct is given by one of the most competent commentators upon the valley of Mexico and the march of the American army. [69] "When an enemy is in front of El Peñon, the communication between it and troops on the other routes is only by way of the city of Mexico itself; in other words, the American troops being at Ayotla, General Santa Anna's forces at El Peñon were one day's march distant from those at Mexicalzingo, three from those under General Valencia, and would have been about four days' march from troops thrown forward on the Chalco route. Fords on these different routes were by no means within supporting distances of each other. Holding the position that General Scott then did, it would have required, of an equal enemy, four times his own force, to have opposed successfully his further advance. The Mexican forces were not numerically equal to this, and, accordingly, they were concentrated at the threatened point. It is evident that as long as the American troops were in front of El Peñon, the enemy necessarily held to their position. In moving off, the former could gain one day the start. This brought the only difficult parts of the Chalco route actually nearer General Scott than the Mexican chief. If to this we add the delay necessary in moving heavy artillery and breaking up from a fortified position, it would seem that, instead of oversight, it was rather impossible for General Santa Anna to meet our forces sooner than he did."
The description of the various routes to the capital has necessarily acquainted the reader with the important Mexican defences on the north, the east, and the north-east of the capital, both by military works hastily thrown up after Santa Anna's retreat from Cerro Gordo, and by the encampment of large bodies of soldiery. We thus, already know a part of the external line of defences at El Peñon, Mexicalzingo, Tezcoco, Santiago Zacualco, and the Pass north of Tenepantla. But in addition to these, there are others that must be noticed on the south and west of the capital, which it should always be recollected is situated in the lap of the valley, but near the western edge of the gigantic rim of mountains.