Treaty of Guadalupe Hidalgo - Background To The War

Background To The War

In Mexico, this is referred to as The American Intervention (La Intervención Estadounidense). Mexico had claimed the area in question for about 25 years since the winning of its independence from the Spanish Empire in 1821 following the Mexican War of Independence. The Spanish Empire had conquered part of the area from the Native American tribes over the preceding three centuries, but there remained rather powerful and independent indigenous nations within that northern region of Mexico. Most of that land was too dry (low rainfall) and too mountainous or hilly to support very much population until the advent of new technology following about 1880: means for damming and distributing water from the few rivers to irrigated farmland; the telegraph; the railroad; the telephone; and electrical power.

There were about 80,000 Mexicans living in the areas of California, New Mexico, Arizona, and Texas during the period of 1845 – 50, and far fewer in Nevada, in southern and western Colorado, and in Utah. On 1 March 1845, U.S. President John Tyler signed legislation to authorize the United States to annex the Republic of Texas, effective on 29 December 1845. The Mexican government, which had never recognized the Republic of Texas as an independent country, had warned that annexation would be viewed as an act of war. The United Kingdom and France, both of which recognized the independence of the Republic of Texas, repeatedly tried to dissuade Mexico from declaring war against its northern neighbor. British efforts to mediate the quandary were fruitless – in part because additional political disputes (particularly the Oregon boundary dispute) arose between Great Britain, as the sovereign of Canada, and the United States.

Before the outbreak of hostilities, on 10 November 1845, President James K. Polk, had sent his envoy, John Slidell, to Mexico to offer the country around $5 million for the territory of Nuevo México, and up to $40 million for Alta California (the present State of California) . The Mexican government dismissed Slidell, refusing to even meet with him. Earlier in that year, Mexico had broken off diplomatic relations with the United States, based partly on its interpretation of the Adams-Onís Treaty of 1819 (which newly independent Mexico claimed it had inherited). In this one the United States had supposedly "renounced forever" all claims to Spanish territory.

Neither side took any further action to avoid a war. Meanwhile Polk settled a major territorial dispute with Britain with the Oregon Treaty, signed on 15 June; this avoided a conflict with Great Britain, and hence gave the U.S. a free hand. After the Thornton Affair of April 25-26, when Mexican forces attacked an American unit in the disputed area with 11 Americans killed, 5 wounded and 49 captured, Congress passed and Polk signed a declaration of war into effect on 13 May 1846. Mexico responded with its war declaration.