On April 11 General Ampudia, the assassin of Sentmanat, arrived at Matamoros to assume the chief command, accompanied by cavalry and followed, as the Americans understood, by two or three thousand more troops. The next day he signalized his advent by ordering Taylor to decamp at once for the other side of the Nueces--a proposal to which a courteous negative was returned--and by compelling all the Americans in the city, "under open threats of violence," to leave town within twenty-four hours. Taylor retaliated by requesting our naval commander off the Rio Grande to stop the use of that stream. As the Mexican attitude made it impossible for us to have the joint navigation implied by our claim, this appeared reasonable; but essentially the measure was defensive, since without supplies coming by water a large force could not remain long at Matamoros. When Ampudia complained, the General pointed out that sealing up the river was only the "natural result of the state of war so much insisted on by the Mexican authorities as actually existing," and offered to reopen it if Ampudia would join him in maintaining an armistice during the negotiations of the two governments; but this led to no result.[7.23] Ampudia's orders and intention had been to attack the Americans as soon as possible, but his glorious prospects darkened immediately. Though given the place of Major General Arista, long at the head of military affairs in this quarter, because he supported the revolution of Paredes while Arista not only frowned upon it, but seemed to aim at making northeastern Mexico independent, Ampudia was detested and thought incompetent--an opinion he did not share--by not a few in the northern army, whereas Arista stood high in his caste, and, as a person of wealth and position, had strong friends well able to make trouble for the central government. Consequently an order dated April 4 made Arista commander-in-chief with Ampudia as lieutenant. The latter was immediately forbidden to shine on the field of glory, and, finding his officers would not support him in disobedience, he submitted.[7.24] Arista, however, bearing instructions dated April 4 to attack the Americans, reached the scene on the 24th, and ordered his cavalry general, Torrejón, to cross above Matamoros with about 1600 men. Hearing a rumor of this movement, Taylor sent Captain Thornton and about sixty dragoons late that afternoon to reconnoitre; and the next morning, some twenty-eight miles from camp, finding himself completely shut in by overwhelming forces, the Captain tried to break through, lost several men killed and wounded, and then with all the rest surrendered. This was war. "Hostilities have begun," announced Arista on the day of his arrival. "Hostilities may now be considered as commenced," reported Taylor on the 26th; and--besides advising Polk to organize twelve-months volunteers--he at once called upon Texas and Louisiana for about 5000 men.[7.25] [REASONS FOR POLK'S COURSE]