As the United States grew, so did hostilities with Mexico to the West.
In 1803, the US bought the Louisiana Territory from the French, 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 included in 1845.
This 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.