When mexico refused to sell california to the US. what did polk do to gain california?

He declared war because he thought he had a reason to start it

Explanation:

After the annexation of the state of Texas, Polk turned his attention to California, hoping to buy the territory from Mexico before any other European state. Polk insisted that Mexico began the war by "shedding American blood on American soil." In 1848 the war was concluded by the Treaty of Guadalupe-Hidalgo. The agreement forced the Mexican government to agree to give the U. S. the territory from Alta California and Santa Fe to Nuevo Mexico. This has added 525,000 square miles to U. S. territory, including land that makes up all or parts of present-day Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming. The United States paid $ 15 million in compensation and canceled $ 3.5 million in debt. Mexico acknowledges the loss of Texas and has accepted the Rio Grande as its northern border. The largest territorial expansion of the United States took place through Polk's annexation of Texas in 1845, the negotiation of the Oregon Treaty with Great Britain in 1846, and the conclusion of the Mexican-American War. The war was viewed as a victory in the United States, but was criticized for its human casualties, monetary costs and heavy hands.