Richard Pakenham was a British diplomat serving in Mexico. In 1841 he wrote to Lord Palmerston, the foreign secretary, urging the British government "to establish an English population in the magnificent Territory of Upper California... no part of the World offering greater natural advantages for the establishment of an English colony... by all means desirable... that California, once ceasing to belong to Mexico, should not fall into the hands of any power but England... daring and adventurous speculators in the United States have already turned their thoughts in this direction."
Politicians in the United States became aware that Britain and France were both interested in taking Texas and California from Mexico. In 1842, Waddy Thompson, Jr., a former member of Congress, argued that the United States should consider taking control of California: "As to Texas I regard it as of very little value compared with California, the richest, the most beautiful and the healthiest country in the world... with the acquisition of Upper California we should have the same ascendency on the Pacific... France and England both have had their eyes upon it."
In July, 1845, the United States Army, under the leadership of Zachary Taylor, arrived in Texas. Talks began with the Mexican government but on 29th December, 1845, James Polk, the president of the United States, announced Texas had become the 28th state. Some member of Congress, including Abraham Lincoln, argued against the action, believing it to be imperialist war of aggression and an attempt to seek new slave states.
General Zachary Taylor defeated the Mexicans at Palo Alto on 8th May, 1846 while General Winfield Scott organised a campaign that involved a seaborne invasion of Mexico that captured Vera Cruz and a march inland to Mexico City, which was captured on 14th September, 1846. Meanwhile General Stephen Kearny conquered New Mexico and with the support of John Fremont took control of California.
Hostilities were terminated by the signing of the Treaty of Guadalupe-Hidalgo in 1848. Under the terms of the treaty the United States gained the territories of California, New Mexico and Utah. The treaty also established that the Rio Grande River marked the definitive boundary line between the two countries. During the war the United States Army lost 13,283 men and had 8,304 wounded.