As the United States grew, so did its hostilities with Mexico.
In 1803, the US bought the Louisiana Territory from the French, essentially doubling its size overnight.
In 1836, the Republic of Texas declared independence from Mexico and applied to join the United States. Initially rejected due to its practice of slavery, it was finally accepted in 1845.
Cessation of Texas started the Mexican-American War.
The war culminated with the fall of Mexico City and the signature of a treaty which granted today's states of California, Nevada, Utah, Colorado, Arizona, New Mexico and Texas to the United States in 1848.
Fun fact : due to limited communication between the East and West coasts, it took 3 months for official confirmation that the war had started to reach California.
California officially joined the United States in 1850 as the 31st state.
In 1846, American settlers had seized a Mexican barrack in Sonoma and raised a flag proclaiming the Californian Republic.
This flag became the basis for the modern Californian flag.