The 1845 annexation of Texas, which continued to claim the land east of the Río Grande, encouraged U.S. expansionists to demand the annexation of all the Southwest and California. After moving U.S. troops to the mouth of the Río Grande, which Mexico considered a provocation, United States President James K. Polk declared war on Mexico in 1846 and sent General Stephen Watts Kearny and the Army of the West to invade New Mexico. Kearny took Santa Fe without firing a shot and claimed New Mexico for the United States on August 18, 1846. The Mexican War ended in 1848 with the Treaty of Guadalupe Hidalgo, which formally ceded New Mexico to the United States.
The Congress of the United States created the Territory of New Mexico under the Compromise Measures of 1850, which were designed to ease the tension between the South and the North over the extension of slavery in the new territories. The compromise divided the area east of California into the territories of New Mexico (now New Mexico and Arizona) and Utah, and both were opened to settlement by both slaveholders and antislavery settlers.
In 1853 President Franklin Pierce sent railroad entrepreneur and diplomat James Gadsden to purchase territory from Mexico to use for a railroad. He reached an agreement for $15 million, which Congress later reduced to $10 million. The purchase added a considerable amount of new land south of the Gila River to New Mexico.
During the American Civil War (1861-1865), New Mexico territory was temporarily controlled by the Confederacy. In July 1861 Texans captured the southern part of the territory, and the Confederacy organized the region south of the 34th parallel, calling it the Territory of Arizona. In 1862 the Confederates briefly captured Santa Fe but then lost their supplies at the Battle of Glorieta and were forced to withdraw from the territory. After the Confederate withdrawal, the U.S. government organized the Arizona Territory in 1863 in the western portion of the Territory of New Mexico, reducing New Mexico to its present boundaries. "New Mexico" © Emmanuel BUCHOT, Encarta, Wikipedia
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