President James Polk was in office right after Congress voted in 1844 to annex Texas into the United States. Polk then moved troops into an area of Texas that the nation of Mexico still claimed, and many people feel that he did so intentionally to provoke the Mexican Government.
Whatever his intention, Mexico became angry and declared War, and the Mexican American War began. Mexico was confident, but their confidence soon faded as they began to lose battle after battle.
Finally, after about a year of war, the United States sat down and worked out a treaty with Mexico to end the war. Some of the things that Mexico sought, such as war reparations and claims from private Mexican citizens against the Mexican government to be paid for by the United States was set up. In addition, Mexico agreed to sell a large amount of land in what is now Utah, California, Nevada, Wyoming, New Mexico, Arizona and parts of Colorado to the United States for $15 million dollars.
The treaty was settled and signed by both Mexico and the United States. President James Polk himself helped work out the details, and he signed for the United States the Treaty of Guadalupe Hidalgo.
At the time it was considered a trade off, many people did not think that Mexico should have been given $15 million dollars, because they were being soundly defeated in each of the military battles of the Mexican American War.
But President James Polk was a pragmatic soul, and he believed in being Fair. In return for the money that was paid to Mexico, the United States was able to declare an end to the War, and received 525,000 square miles of land.
This dovetailed with President Polk’s wishes for expansion, and the money supported Mexico and helped make up for some of their losses during the War with the United States.