Why did the United States fear Texas expansion to California?

Conflict with Mexico began when the United States annexed Texas as a state in 1845.

Mexico claimed that the new border between Texas and Mexico was the Nueces River, while the United States contested the border was the Rio Grande.

Fighting began when a detachment of U. S. cavalry was attacked near the Rio Grande.

Zachary Taylor and Winfield Scott led armies to a series of military successes that culminated in the capture of Mexico City in 1847.

The 1848 Treaty of Guadalupe Hidalgo, forced onto the remnant Mexican government, ended the war and enforced the Mexican Cession of the northern territories of Alta California and Santa Fe de Nuevo México to the United States. The U. S. agreed to pay $15 million compensation for the physical damage of the war and assumed $3.25 million of debt already owed earlier by the Mexican government to U. S. citizens. Mexico acknowledged the loss of what became the State of Texas and accepted the Rio Grande as its northern border with the United States.