Santa Fe de Nuevo México in the context of "Mexican Cession"

The Southwestern United States, also known as the American Southwest or simply the Southwest, is a geographic and cultural region of the United States that includes Arizona and New Mexico, along with adjacent portions of California, Colorado, Nevada, Oklahoma, Texas, and Utah. The largest cities by metropolitan area are Phoenix, Las Vegas, El Paso, Albuquerque, and Tucson. Before 1848, in the historical region of Santa Fe de Nuevo México as well as parts of Alta California and Coahuila y Tejas, settlement was almost non-existent outside of New Mexico's pueblos and Spanish or Mexican municipalities. Much of the area had been a part of New Spain and Mexico until the United States acquired the area through the Treaty of Guadalupe Hidalgo in 1848 and the smaller Gadsden Purchase in 1854.

While the region's boundaries are not officially defined, there have been attempts to do so. One such definition is from the Mojave Desert in California in the west (117° west longitude) to Carlsbad, New Mexico, in the east (104° west longitude); another says that it extends from the Mexico–United States border in the south to the southern areas of Colorado, Utah, and Nevada in the north (39° north latitude). In another definition, the core Southwestern U.S. includes only the states of Arizona and New Mexico; others focus on the land within the old Spanish and Mexican borders of the Nuevo México Province or the later American New Mexico Territory.

Colorado is a landlocked state in the Western United States. It is one of the Mountain states, and Southwestern region of the United States, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas to the east, and Oklahoma to the southeast. Colorado is noted for its landscape of mountains, forests, high plains, mesas, canyons, plateaus, rivers, and desert lands. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains. Colorado is the eighth-largest U.S. state by area and the 21st by population. The United States Census Bureau estimated the population of Colorado to be 5,957,493 as of July 1, 2024, a 3.2% increase from the 2020 United States census.

The region has been inhabited by Native Americans and their Paleo-Indian ancestors for at least 13,500 years and possibly much longer. The eastern edge of the Rocky Mountains was a major migration route for early peoples who spread throughout the Americas. In 1848, much of the Nuevo México region was annexed to the United States with the Treaty of Guadalupe Hidalgo. The Pike's Peak Gold Rush of 1858–1862 created an influx of settlers. On February 28, 1861, U.S. President James Buchanan signed an act creating the Territory of Colorado, and on August 1, 1876, President Ulysses S. Grant signed Proclamation 230, admitting Colorado to the Union as the 38th state. The Spanish adjective "colorado" means "colored red" or "ruddy". Colorado is nicknamed the "Centennial State" because it became a state 100 years (and four weeks) after the signing of the United States Declaration of Independence.

Arizona is a landlocked state in the Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the northwest and California to the west, and shares an international border with the Mexican states of Sonora and Baja California to the south and southwest. Its capital and largest city is Phoenix, which is the most populous state capital and fifth-most populous city in the United States. Arizona is divided into 15 counties.

Arizona is the sixth-largest state by area and the 14th-most-populous of the 50 states. It is the 48th state and last of the contiguous states to be admitted to the Union, achieving statehood on February 14, 1912. Historically part of the territory of Alta California and Nuevo México in New Spain, it became part of independent Mexico in 1821. After being defeated in the Mexican–American War, Mexico ceded much of this territory to the United States in 1848, where the area became part of the New Mexico Territory. The southernmost portion of the state was acquired in 1853 through the Gadsden Purchase.

New Mexico is a landlocked state in the Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also borders the state of Texas to the east and southeast, Oklahoma to the northeast, and shares an international border with the Mexican states of Chihuahua and Sonora to the south. New Mexico's largest city is Albuquerque, and its state capital is Santa Fe, the oldest state capital in the U.S.—founded in 1610 as the government seat of Nuevo México in New Spain—and the highest in elevation, at 6,998 feet (2,133 m).

New Mexico is the fifth-largest of the fifty states by area, but with just over 2.1 million residents, ranks 36th in population and 45th in population density. Its climate and geography are highly varied, ranging from forested mountains to sparse deserts; the northern and eastern regions exhibit a colder alpine climate, while the west and south are warmer and more arid. The Rio Grande and its fertile valley runs from north-to-south, creating a riparian biome through the center of the state that supports a bosque habitat and distinct Albuquerque Basin climate. One–third of New Mexico's land is federally owned, including many protected wilderness areas, 15 national parks and monuments, and three UNESCO World Heritage Sites wholly contained within the state—the most of any U.S. state.

Albuquerque, also known as ABQ, Burque, Duke City, or the Q, is the most populous city in the U.S. state of New Mexico, and the county seat of Bernalillo County. Founded in 1706 as La Villa de Alburquerque by Santa Fe de Nuevo México governor Francisco Cuervo y Valdés, and named in honor of Francisco Fernández de la Cueva, 10th Duke of Alburquerque and Viceroy of New Spain, it was an outpost on El Camino Real, linking Mexico City to the northernmost territories of New Spain.

Located in the Albuquerque Basin, the city is flanked by the Sandia Mountains to the east and the West Mesa to the west, with the Rio Grande and its bosque flowing north-to-south through the middle. According to the 2020 census, Albuquerque had 564,559 residents, making it the 32nd most populous city in the U.S. and the fourth largest in the Southwest. The Albuquerque metropolitan area had 955,000 residents in 2023, and forms part of the Albuquerque–Santa Fe–Los Alamos combined statistical area, which had a population of 1,162,523.

Pueblo refers to the settlements of the Pueblo peoples, Native American tribes in the Southwestern United States, currently in New Mexico, Arizona, and Texas. The permanent communities, including some of the oldest continually occupied settlements in the United States, are called pueblos (lowercased).

Spanish explorers of northern New Spain used the term pueblo to refer to permanent Indigenous towns they found in the region, mainly in New Mexico and parts of Arizona, in the former province of Nuevo México. This term continued to be used to describe the communities housed in apartment structures built of stone, adobe, and other local material. The structures were usually multistoried buildings surrounding an open plaza. Many rooms were accessible only through ladders raised and lowered by the inhabitants, thus protecting them from break-ins and unwanted guests. Larger pueblos are occupied by hundreds to thousands of Puebloan people.

The Territory of New Mexico was an organized incorporated territory of the United States from September 9, 1850, until January 6, 1912. It was created from the U.S. provisional government of New Mexico, as a result of Nuevo México becoming part of the American frontier after the Treaty of Guadalupe Hidalgo. It existed with varying boundaries until the territory was admitted to the Union as the U.S. state of New Mexico in 1912. This jurisdiction was an organized, incorporated territory of the US for nearly 62 years, the longest period of any territory in the contiguous United States.