Not many realized, at the moment, that the extension of the sovereignty of the United States to the Pacific above the forty-second parallel of north latitude would require the like extension to the south of it. Once across the Rockies it was inevitable that the natural boundary in the southwest, as well as in the northwest, should be ultimately attained. It came sooner than anybody expected. {327} CHAPTER XVI. THE WAR WITH MEXICO Slidell's Mission to Mexico--The Failure of the Mission--The Concentration of the Mexican Forces at Matamoras--The United States Forces Ordered to the Rio Grande--Hostilities Opened--The Battles of Palo Alto and Resaca de la Palma--The Attitude of Congress toward the War--Congressional Approval of the War--The Occupation of New Mexico and Upper California, and the Advance into Mexico--California's Importance--The Battle of Buena Vista--Vera Cruz and Cerro Gordo--Contreras, San Antonio, and Cherubusco--The Plan for a Cession of Territory from Mexico--The Wilmot Proviso--The Fate of the Wilmot Proviso in the Senate--The Proviso Again Voted by the House of Representatives--The Upham Amendment in the Senate--The Amendment Defeated by the Efforts of Mr. Cass--The Wilmot Proviso Dropped in the House--The Mission of Mr. Trist--Rejection of Mr. Trist's Propositions by the Mexicans--Negotiations Broken off--Molino del Rey, Chapultepec, Mexico--The Recall of Mr. Trist, and the Treaty of Guadalupe Hidalgo--Ratification of the Treaty. [Sidenote: Slidell's mission to Mexico.] As we have seen, the Mexican Government had announced to the Government of the United States that the annexation of Texas would be regarded by Mexico as a _casus belli_. Consequently, as soon as the matter was concluded, the Mexican Envoy left Washington, and all diplomatic relations between the two Powers were suspended. Some six months later, President Polk made overtures for the resumption of these relations, and, upon meeting a somewhat friendly response, commissioned Mr. John Slidell, of Louisiana, to go to Mexico and negotiate a treaty, which should settle all the differences between the two Powers. {328} Mr. Slidell arrived at Vera Cruz on November 30th, 1845, and found that President Herrera's Administration was afraid to receive him, because the military or war party in Mexico, led by General Paredes, was greatly excited by the attitude of the Administration toward the United States, and threatened revolution. The Mexican Government actually refused audience to Mr. Slidell, on December 21st. [Sidenote: The failure of the mission.] Before the end of the year, President Herrera gave way to General Paredes, who assumed the presidency of the Republic, and, under direction from President Polk, Mr. Slidell announced himself to the new Administration. This was March 1st, 1846. On the 12th, he received the refusal of the Paredes Government to give him audience, and immediately left Mexico.