Captain John C. Frémont, leading a U.S. Army topographical expedition to survey the Great Basin, had entered the Sacramento Valley in December 1845.

Frémont's party was at Upper Klamath Lake, Oregon Territory, when it received word that war between Mexico and the U.S. was imminent; the party had then returned to Alta California.

Frémont has reportedly begun agitating American settlers to revolt against Mexican rule.

After telling the Mexican governor and the American Consul Larkin he is merely buying supplies on the way to Oregon, he instead goes to the populated area of California and visits Santa Cruz and the Salinas Valley, explaining he has been looking for a seaside home for his mother.

Mexican authorities become alarmed and order him to leave.

Frémont responds by building a fort on Gavilan Peak and raising the American flag.

Larkin sends word that Frémont's actions are counterproductive.

Frémont leaves California in March but will return to California and take control of the California Battalion following the outbreak of the Bear Flag Revolt in Sonoma.

Newly elected U.S. President James K. Polk had made a proposition in 1845 to purchase Alta California and Santa Fe de Nuevo México from Mexico, and to agree upon the Rio Grande river as the southern border of United States.

When that offer was rejected, President Polk moves U.S. troops commanded by Major General Zachary Taylor further south into the disputed Nueces Strip.

The border of Texas as an independent state was originally never settled.

The Republic of Texas claims land up to the Rio Grande based on the Treaties of Velasco, but Mexico refuses to accept these as valid, claiming that the Rio Grande in the treaty is the Nueces River, and refers to the Rio Grande as the Rio Bravo.

The ill-fated Texan Santa Fe Expedition of 1841 had attempted to realize the claim to New Mexican territory east of the Rio Grande, but its members had been captured and imprisoned.

Reference to the Rio Grande boundary of Texas had been omitted from the U.S. Congress's annexation resolution to help secure passage after the annexation treaty failed in the Senate.

President Polk claims the Rio Grande boundary, and when Mexico sends forces over the Rio Grande, this provokes a dispute.

In July 1845, Polk had sent General Zachary Taylor to Texas, and by October thirty-five hundred Americans are on the Nueces River, ready to take by force the disputed land.

Polk wants to protect the border and also covets for the U.S. the continent clear to the Pacific Ocean.

At the same time Polk writes to Thomas O. Larkin, the American consul in the Mexican territory of Alta California, disclaiming American ambitions in California, but offering to support independence from Mexico or voluntary accession to the United States, and warning that the United States would oppose a British or French takeover.

To end another war scare with the United Kingdom over the Oregon Country, Polk had signed the Oregon Treaty dividing the territory, angering northern Democrats who feel he is prioritizing Southern expansion over Northern expansion.

it had taken almost three months for definitive word of Congress' declaration of war against Mexico on May 13, 1846, to get to California.

American consul Thomas O. Larkin, stationed in Monterey, works successfully during the events in that vicinity to avoid bloodshed between Americans and the Mexican military garrison commanded by General José Castro, the senior military officer in California.