Alta California in the context of "Southwestern United States"

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.

The Mexican Cession (Spanish: Cesión mexicana) is the territory that Mexico ceded to the United States in the Treaty of Guadalupe Hidalgo in 1848 after the Mexican–American War. It comprises the states of California, Texas, New Mexico, Utah, Nevada, and Arizona, and parts of Colorado, Oklahoma, Kansas, and Wyoming in the present-day Western United States. Consisting of roughly 529,000 square miles (1,370,000 km), not including Texas, the Mexican Cession was the third-largest acquisition of territory in U.S. history, surpassed only by the 827,000-square-mile (2,140,000 km) Louisiana Purchase of 1803 and the later 586,000-square-mile (1,520,000 km) Alaska Purchase from Russia in 1867.

Most of the ceded territory had not been claimed by the Republic of Texas following its de facto independence in the 1836 revolution. Texas had only claimed areas east of the Rio Grande. After annexation into the U.S. and admission as a state in 1845, Texas's southern and western boundaries with the Mexican state of Santa Fe de Nuevo México had not been defined by the U.S. Most of the ceded area was part of the Mexican province of Alta California (Upper California) while the southeastern strip east of the Rio Grande had been part of the state of Santa Fe de Nuevo México. Mexico had controlled the ceded territories with considerable local autonomy. After the Mexican War of Independence finished in 1821, the area had seen several revolts but the Mexican government sent few troops from central Mexico to suppress them. In 1846, U.S. President James K. Polk sent soldiers to occupy the disputed territory between the Nueces River and the Rio Grande, starting the Mexican–American War. At the war's outbreak, the U.S. conducted a naval landing on Mexico's northeastern coast on the Gulf of Mexico, and advanced into Nuevo México.

The Portolá expedition was a Spanish voyage of exploration in 1769–1770 that was the first recorded European exploration of the interior of the present-day California. It was led by Gaspar de Portolá, governor of Las Californias, the Spanish colonial province that included California, Baja California, and other parts of present-day Mexico and the United States. The expedition led to the founding of Alta California and contributed to the solidification of Spanish territorial claims in the disputed and unexplored regions along the Pacific coast of North America.

Sacramento is the capital city of the U.S. state of California. The county seat of Sacramento County, it is located at the confluence of the Sacramento and American Rivers in the Sacramento Valley. It is the fourth-most populous city in Northern California, sixth-most populous city in the state, and 35th-most populous city in the United States, with a population of 524,943 at the 2020 census. The Sacramento metropolitan area, with 2.46 million residents, is the 27th-largest metropolitan area in the country.

Before the arrival of the Spanish, the area was inhabited by the Nisenan, Maidu, and other indigenous peoples of California. In 1808, Spanish cavalryman Gabriel Moraga surveyed and named the Río del Santísimo Sacramento (Most Holy Sacrament River), after the Blessed Sacrament. In 1839, Juan Bautista Alvarado, Mexican governor of Alta California, granted the responsibility of colonizing the Sacramento Valley to Swiss-born Mexican citizen John Augustus Sutter, who subsequently established Sutter's Fort and the settlement at the Rancho Nueva Helvetia. Following the American Conquest of California and the 1848 Treaty of Guadalupe-Hidalgo, the waterfront developed by Sutter began to be developed and incorporated in 1850 as the City of Sacramento. In 1852, the city offered its county courthouse to the state of California to house the state legislature, resulting in the city becoming the permanent state capital in 1854 and ushering in the construction of a new state capitol building which was finished in 1874.

San Diego (/ˌsæn diˈeɪɡoʊ/ SAN dee-AY-goh; Spanish: [san ˈdjeɣo]) is a city on the Pacific coast of Southern California, adjacent to the Mexico–United States border. It is the eighth-most populous city in the U.S. and second-most populous city in California with a population of over 1.4 million, while the San Diego metropolitan area with over 3.3 million residents is the 18th-largest metropolitan area in the country. San Diego is the county seat of San Diego County. It is known for its mild Mediterranean climate, extensive beaches and parks, long association with the United States Navy, and recent emergence as a wireless, electronics, healthcare, and biotechnology development center.

Historically home to the Kumeyaay people, San Diego has been referred to as the Birthplace of California, as it was the first site visited and settled by Europeans on what is now the West Coast of the United States. In 1542, Juan Rodríguez Cabrillo claimed the area for Spain, forming the basis for the settlement of Alta California, 200 years later. The Presidio and Mission San Diego de Alcalá, founded in 1769, formed the first European settlement in what is now California. In 1821, San Diego became part of the newly declared Mexican Empire. California was ceded to the U.S. in 1848 following the Mexican–American War and was admitted as the 31st state in 1850.

The Conquest of California, also known as the Conquest of Alta California or the California Campaign, was a military campaign during the Mexican–American War carried out by the United States in Alta California (modern-day California), then part of Mexico, lasting from 1846 to 1847, and ending with signing of the Treaty of Cahuenga by military leaders from both the Californios and Americans.

The Oregon boundary dispute or the Oregon Question was a 19th-century territorial dispute over the political division of the Pacific Northwest of North America between several nations that had competing territorial and commercial aspirations in the region.

Expansionist competition into the region began in the 18th century, with participants including the Russian Empire, Great Britain, Spain, and the United States. After the War of 1812, the Oregon dispute took on increased importance for diplomatic relations between the British Empire and the fledgling American republic. In the mid-1820s, the Russians signed the Russo-American Treaty of 1824 and the Russo-British Treaty of 1825, and the Spanish signed the Adams–Onís Treaty of 1819, by which Russia and Spain formally withdrew their respective territorial claims in the region, and the British and the Americans acquired residual territorial rights in the disputed area. But the question of sovereignty over a portion of the North American Pacific coast was still contested between the United Kingdom and the United States. The disputed area was defined as the region west of the Continental Divide of the Americas, north of Mexico's Alta California border of 42nd parallel north, and south of Russian America at parallel 54°40′ north. The British generally called this region the Columbia District and the Americans generally called it Oregon Country.