The army of occupation in Texas, commanded by General Zachary Taylor, had, during the preceding winter, been moving westward with the view of encamping in the valley of the Rio Grande. On the 28th of March General Tyler took up his position on the banks of the river, opposite Matamoros, and strengthened himself by the erection field-works. General Ampudia, in command of the Mexican army stationed at Matamoros, was highly excited by the arrival of the American army, and on the 12th of April notified General Taylor to break up his camp within twenty-four hours, and to retire beyond the Nueces River. In the event of his failure to comply with these demands, Ampudia announced that "arms, and arms alone, must decide the question." According to the persistent claim of the Mexican Government, the Nueces River was the western boundary of Texas; and the territory between that river and the Rio Grande--a breadth of one hundred and fifty miles on the coast--was held by Mexico to be a part of her domain, and General Taylor consequently an invader of her soil. No reply was made to Ampudia; and on the 24th of April General Arista, who had succeeded to the command of the Mexican army, advised General Taylor that "he considered hostilities commenced, and should prosecute them." BEGINNING OF MEXICAN WAR. Directly after this notification was received, General Taylor dispatched a party of dragoons, sixty-three in number, officers and men, up the valley of the Rio Grande, to ascertain whether the Mexicans had crossed the river. They encountered a force much larger than their own, and after a short engagement, in which some seventeen were killed and wounded, the Americans were surrounded, and compelled to surrender. When intelligence of this affair reached the United States, the war-spirit rose high among the people. "Our country has been invaded," and "American blood spilled on American soil," were the cries heard on every side. In the very height of this first excitement, without waiting to know whether the Mexican Government would avow or disavow the hostile act, President Polk, on the 11th of May, sent a most aggressive message to Congress, "invoking its prompt action to recognize the existence of war, and to place at the disposition of the Executive the means of prosecuting the contest with vigor, and thus hastening the restoration of peace." As soon as the message was read in the House, a bill was introduced authorizing the President to call out a force of fifty thousand men, and giving him all the requisite power to organize, arm, and equip them. The preamble declared that "war existed by the act of Mexico," and this gave rise to an animated and somewhat angry discussion. The Whigs felt that they were placed in an embarrassing attitude. They must either vote for what they did not believe, or, by voting against the bill, incur the odium