structural:expansion
Polk's Actions

In July 1845, Polk sent General Zachary Taylor to Texas, and by October, Taylor commanded 3,500 Americans on the Nueces River, ready to take by force the disputed land. At the same time, President Polk wrote to 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 any European attempts to take over.

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

In the winter of 1845–46, the federally commissioned explorer John C. Frémont and a group of armed men appeared in Alta California. After telling both the Mexican governor and the American Consul Thomas O. Larkin that he was merely buying supplies on the way to Oregon, he instead went to the populated area of California and visited Santa Cruz and the Salinas Valley, explaining he had been looking for a seaside home for his mother. Mexican authorities became alarmed and ordered him to leave. Frémont responded by building a fort on Gavilan Peak and raising the American flag. Larkin sent word that Frémont's actions were counterproductive. Frémont left California in March but returned and took control of the California Battalion following the outbreak of the Bear Flag Revolt in Sonoma.

In November 1845, Polk sent John Slidell, a secret representative, to Mexico City with an offer to the Mexican government of $25 million for the Rio Grande border in Texas and Mexico's provinces of Alta California and Santa Fe de Nuevo México. U.S. expansionists wanted California to thwart any British interests in the area and to gain a port on the Pacific Ocean. Polk authorized Slidell to forgive the $3 million owed to U.S. citizens for damages caused by the Mexican War of Independence and pay another $25 to $30 million for the two territories.