[Sidenote: The concentration of the Mexican forces at Matamoras.] During the summer of 1845, the Mexican Government had begun to collect troops and munitions of war at Matamoras, on the south bank of the Rio Grande near its mouth. The purpose of all this was without question an expedition across the Rio Grande, and into the region north of it. By an act of the Texan Congress, of December 19th, 1836, the Rio Grande was designated as the southwestern boundary of Texas. The United States took Texas with this boundary, reserving in the resolution of annexation the right of adjusting the Texan boundaries with foreign states. This meant, of course, that the United States might change the boundary which Texas had given herself, as the result of her successful rebellion, her revolution, against Mexico, by an agreement with Mexico, in so far as Texas was concerned. It further meant that any such change must be made either by an act of the Congress of the United States or by a treaty between the United States and Mexico. Until, {329} however, this adjustment should take place, it was the duty of the President of the United States to defend the boundary with which Texas came into the Union. Moreover, Congress had passed an act, on December 31st, 1845, in which Corpus Christi, a town situated on the south side of the river Nueces, was made an United States port of delivery. The town was, also, the head-quarters of the United States army in Texas, and had been so from the period of annexation. [Sidenote: The United States forces ordered to the Rio Grande.] When now the Mexican Government refused to receive Mr. Slidell, and continued to increase the forces at Matamoras, President Polk felt it to be his duty to defend the line of the Rio Grande. On January 13th, he ordered General Taylor, then in command at Corpus Christi, to advance to the northern bank of the Rio Grande. The General, with his little army of about 2,000 men, arrived upon the Rio Grande, at a point opposite Matamoras, on March 28th, and began fortifying his position. [Sidenote: Hostilities opened.] On April 12th, the Mexican commander, General Ampudia, demanded the withdrawal of Taylor's forces within twenty-four hours, and their retirement across the Nueces, under threat of the appeal to arms. Taylor paid no attention to the demand, and, on the 24th, he received notice from General Arista, the successor of Ampudia, that hostilities were opened. [Sidenote: The battles of Palo Alto and Resaca de la Palma.]