The populated coastal region of the territory was admitted into the Union in 1850 as the State of California, while the vast, sparsely populated interior region would only later gain statehood as Nevada, Utah, and parts of New Mexico, Arizona, Wyoming, and Colorado.

Afterwards, when its peninsular character was ascertained, it was called simply California; but the territory so designated was unlimited in extent.

When the expeditions for the settlement of San Diego and Monterey marched, it was understood that they were going, not out of California, but into a new part of it.

[16]The first attempted Spanish occupation of California was by the Jesuit missionary Eusebio Kino, in 1683.

The Jesuits went on to found a total of 18 missions in the lower two-thirds of the Baja California Peninsula.

At the same time, a new visitador, José de Gálvez, was dispatched from Spain with authority to organize and expand the fledgling province.

Expansion came through exploration and colonization expeditions led by Portolá (1769), his successor Pedro Fages (1770), Juan Bautista de Anza (1774–76), the Franciscan missionaries and others.

Independent Mexico retained the division but demoted the former provinces to territories, due to populations too small for statehood.

Following Mexico's defeat in the war, most of the former Alta California territory was ceded on 2 February 1848 to the United States, under the terms of the Treaty of Guadalupe Hidalgo.

The areas acquired by the U.S. remained under military authority, pending creation of civilian government through territorial designation and/or statehood.

The Gadsden Purchase transferred addition territory from Mexico to the United States.

Inland regions were mostly unexplored by the Spanish, leaving them generally outside the control of the colonial authorities.

The eastern border of upper Las Californias was never officially defined under either Spanish or subsequent Mexican rule.