The shifting borders of the Californias: from a mythical island to two nations

Countries & Cities · Published 12 October 2026 Swipe more like this →

In short: Once thought to be a cluster of islands, the region known as the Californias has undergone radical transformations. From a vast Spanish colonial province to a territory split between the United and Mexican states, its borders have been redrawn by wars, treaties, and shifting imperial ambitions.

In the early era of Spanish exploration, the region was often referred to by the plural name 'Las Californias.' This stemmed from a widespread geographical misconception that the territory was an island, or even a series of islands. As explorers confirmed the peninsular nature of the land, the name shifted toward 'California,' though the term 'Las Californias' was later revived to distinguish between the established 'Old California' (Antigua) and the unexplored northern frontiers of 'New California' (Nueva).

The administrative landscape was highly fluid. In 1768, a joint dispatch from Viceroy de Croix and José de Gálvez formally established the ambitious name 'Las Californias.' By 1804, the single province was split into Alta and Baja California. While the Mexican government briefly reunited them as a single department in 1836 under the 'Siete Leyes' reforms, the Mexican–American War (1846–48) fundamentally altered the map. Following the Treaty of Guadalupe Hidalgo on February 2, 1848, the vast territory of Alta California was ceded to the United States.

This geopolitical shift saw the coastal region of Alta California admitted as the 31st U.S. state in 1850. Meanwhile, the interior regions eventually formed the foundations for several other states, including Nevada, Utah, and Arizona. Mexico split the Baja California region into two distinct territories during 1931, which subsequently evolved into the states of Baja California and Baja California Sur in 1952 and 1974. Today, the 'Californias' remains a collective term for these three interconnected states, linked by shared history and economic ties.

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