After Congress finally approved the funds–minus the Wilmot Proviso–to purchase the western territories. Polk chose State Department clerk Nicholas Trist to deliver an offer of three million dollars as a downpayment for California and New Mexico to the Mexican government. Trist had no authority to sign a treaty but was merely meant to make the offer. When Trist arrived on May 6, the army commanded by Winfield Scott was closing in on Mexico City, but Scott refused to allow the clerk to talk with the Mexican government, feeling that such negotiations were his alone. After a long exchange of bitter letters back and forth, the two men resolved their differences after Trist fell ill and the General sent him a jar of marmalade! They worked well together from that point on. Scott, though, realized that Trist was his ticket to the Whig nomination–and in order to win the nomination, he would have to become a bigger hero than Taylor.
Polk received no word of Scott and Trist's joint secret actions until September 14. Scott had brought the army right up to the gates of Mexico City and then sent Trist in to negotiate. Polk angrily ordered Scott to lift the truce and attack and he recalled Trist. On that same day, though, Scott took Mexico City. Over the coming weeks, Polk repeatedly recalled Trist and later recalled Scott as well. In his third message to Congress, on December 7, 1847, Polk outlined his plan to buy California and New Mexico from Mexico to prevent it from falling into European hands. Congress balked, and a House resolution condemned the president and the war.
On February 19, a special messenger arrived with the treaty that Trist had signed with Mexico. For fifteen million dollars, the U.S. received almost all of the land originally demanded. Although he disliked Trist, the treaty was acceptable and Polk announced his support. On March 10, Congress ratified the peace treaty.
Polk's goal of "Manifest Destiny" had been achieved. He had expanded the U.S. all the way to the Pacific Ocean. He would always be remembered as the most expansionist president in American history. Beyond that, however, he enforced the Monroe Doctrine throughout his tenure. When, in 1848, it appeared that the Yucatan Peninsula might become a British colony, Polk iterated the Doctrine and helped make it a cornerstone of American diplomacy.
As the war ate up most of President Polk's time, he and his congressional supporters continued work on many of the domestic proposals Polk favored. His reform of the postal service passed with relatively little debate. And, after years of legislative wrangling, Congress finally granted the sub-treasury for which Martin Van Buren had advocated so many years before.
As for his promises to lower tariffs, Polk supported a tariff plan written by his secretary of the treasury, Robert J. Walker. Polk, as a Democrat, had long opposed any tariffs, whereas the Whigs favored the protection the tariffs offered. Walker drew up a new set of tariffs with "protection incident but not the object," and with Polk's backing helped secure Western support by pointing out that the lower tariffs would allow farmers to sell grain overseas. The passage of the Walker tariff was instrumental in the opening of free trade with England and in causing the English to open its markets to U.S. grain.