What were the limitations on further US expansion at that war? why did they stop on the bank of the Rio Grande and in particular what stopped them from taking over Baja California?
"California" (at the time), consisted of three distinct regions:
1) A decidedly "Anglo" dominated area around the then-capital at Sonoma (modern northern California).
2) A "mixed" Anglo and Spanish-speaking area in the center (around Los Angeles),
3) A purely Spanish-speaking area in the south (the modern Baja California and Baja California Sur.
During the Mexican-American war, the "Americans" won decisively in the north, but fought a seesaw battle with the Mexicans around Los Angeles. When it came time to make peace, the U.S. Mexican boundary was drawn along a generally northwesterly "line" following the course of the Rio Grande and other rivers that made natural boundaries. The western end of the line started at the source of the Gila River (in modern Arizona), and extended westward to the Pacific Ocean, neatly dividing the "mixed" and Spanish speaking areas (2 and 3 above) to the north and south.
America "purchased" the land north of the border for $15 million, and later paid another $15 million for the Gadsden Purchase that "straightened" the southern borders of Arizona and New Mexico and allowed a railroad to be built through the territory.
In every army there are men who will fight for their country, and, in the case at hand, there was a Captain named Manuel Pineda who fought for Mexico and its anti-slavery constitution of 1824. With a small force of Mexican soldiers and poorly armed routed a superior English/American (remember all thirteen original colonies were English and then called themselves Americans) force in several battles in Baja California. Later, after a tough fight and out of supplies and ammunition he and his men were captured,but the United States learning about the earlier Mexican victory agreed to the border at what is now San Ysidro, San Diego County.
If the U.S. had learned of the defeat of Manuel Pineda and his troops Baja California would be part of the United States, based on our history I have no doubt of that.
As stated by Professor Gutierrez who ever wins the war writes the historical accounts making sure they leave out hero's such as MANUEL PINEDA. However, history has a way of catching up and noting how and who was involved in the war, the incident, and the low down,selfish tactics used to overcome an enemy, and then take his land.
My best, Chuck Pineda; 8th United States Army veteran.