Lol. Don't you mean the Americans living in California declared their independence from Mexico?
Here's the flag of the newly independent California:
And the US didn't grab anything. Mexico ceded the territories following their loss in a war they started.
It was a good deal for Mexico considering their nation was almost completely conquered.
"Outnumbered militarily and with many of its large cities occupied, Mexico could not defend itself; the country was also faced with many internal divisions, inclucing the Caste War of Yucatán. The Treaty of Guadalupe Hidalgo, signed on February 2, 1848, by American diplomat Nicholas Trist and Mexican plenipotentiary representatives Luis G. Cuevas, Bernardo Couto, and Miguel Atristain, ended the war. The treaty gave the U.S. undisputed control of Texas, established the U.S.-Mexican border of the Rio Grande, and ceded to the United States the present-day states of California, Nevada, Utah, New Mexico, most of Arizona and Colorado, and parts of Texas, Oklahoma, Kansas, and Wyoming. In return, Mexico received $15,000,000[56] ($497,452,885 today) – less than half the amount the U.S. had attempted to offer Mexico for the land before the opening of hostilities[57] – and the U.S. agreed to assume $3.25 million ($88,587,500 today) in debts that the Mexican government owed to U.S. citizens.[17]