American strategy was based on a three-pronged attack. Colonel Stephen Kearny (1794-1848) had the task of securing New Mexico, while naval forces under Commodore John D. Sloat (1781-1867) blockaded California and General Zachary Taylor (1784-1850) invaded Tamaulipas, in present-day northern Mexico. In less than two months, Kearny marched his 1700-man army more than a thousand miles, occupied Santa Fe, and declared New Mexico's 80,000 inhabitants American citizens. In California's Sacramento Valley, American settlers revolted even before reliable reports of war had arrived. By January 1847, U.S. naval and ground forces brought California under American control. Meanwhile, the main U.S. army under Taylor took Matamoros and Monterrey.
Although the American invasion of Mexico's northernmost provinces was completely successful, the Mexican government refused to surrender or negotiate. Switching strategies, President Polk ordered General Winfield Scott (1786-1866) to invade central Mexico from the sea, at Veracruz on the Gulf of Mexico, to march inland, and capture Mexico City. Zachary Taylor was in Monterrey when he heard reports that Scott's army had captured the Mexican capital. In this letter Taylor, a Louisiana slaveholder who had never voted in a presidential election, discusses early American military successes, the possibility that he might be nominated for the presidency, and an explosive controversy that had erupted in Congress over the Wilmot Proviso. The proviso, an amendment to a military appropriation bill to prohibit slavery from any territory acquired from Mexico, ultimately passed the House of Representatives but was defeated in the Senate.
I have not heard any of the particulars as regards...Scott's taking possession of the City of Mexico. All we have heard relative to that affair is that he was in quiet possession of that piece & that the Mexican army had dispersed; I presume there will hardly be another battle; that Santa Anna had gone no one knew where, in the direction of the Pacific, & it was supposed he would leave this country; this is Mexican views & Mexican conjecture.... I presume atrocity of some kind or other will grow out of our taking the city & laying it under contribution [occupation], which the Mexicans say has been done, & should they acquiesce in considerable [loss] of territory, it will produce great strife in the streets, when atrocity is laid before that body for their action. The Wilmot Proviso will shake that body to its center... but I hope some compromise will be entered into between the two parties slavery & anti slavery which will have the effect of allying violent passions on both sides, which will have the effect of perpetuating...or shortening the Union....
It is to me a matter of perfect indifference whether I am even elected [as president] or not. I do not intend any party shall use me as a convenience; if drafted I intend to stand aloof, & let Whigs and Democrats [use] this matter in their own way.... Depend on it there will be great changes in the complexion of political affairs between now & the end of this next session of Congress; the Whigs as a party between ourselves I look upon as doomed.... I am gratified I took the position I did, which was not to be the exclusive candidate of any party; & if I am elected at all, it will be by a union of a portion of Whig Democrat and native votes. At any rate I am occupying a position & shall continue to do so. I hope that if not elected, I shall neither be mortified or disappointed.