Partition and secession in California

California, the most populous state in the United States and third largest in area, has been the subject of more than 220 proposals to divide it into multiple states since its admission to the United States in 1850 including at least 27 significant proposals in the first 150 years of statehood. In addition, there have been some calls for the secession of certain states or large regions in the American West (such as the proposal of Cascadia) which often include parts of Northern California. One-third of California residents in a 2016–2017 poll supported peacefully seceding from the United States, up from 20% since 2014.

California was partitioned in its past. What under Spanish rule was called the Province of the Californias (1768–1804) was divided into Alta California (Upper California) and Baja California (Lower California) in 1804 at the line separating the Franciscan missions in the north from the Dominican missions in the south.

After the Mexican–American War, Alta California was admitted to the United States as the present-day State of California. Baja California remained under Mexican rule.

In 1888, under the government of President Porfirio Díaz, Baja California became a federally administered territory called the North Territory of Baja California ("north territory" because it was the northernmost territory in the Republic of Mexico). In 1952, the northern portion of this territory (above 28°N) became the 29th state of Mexico, called Baja California; the sparsely populated southern portion remained a federally administered territory. In 1974, it became the 31st state of Mexico, admitted as Baja California Sur.

The territory that became the present state of California was acquired by the U.S. as a result of the Mexican–American War and subsequent 1848 Mexican Cession. After the war, a confrontation erupted between the slave states of the South and the free states of the North regarding the status of these acquired territories. Among the disputes, the South wanted to extend the Missouri Compromise line (36°30' parallel north), and thus slave territory, west to Southern California and to the Pacific coast, while the North did not.

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