Where was the border between Mexico and America before the Mexican War?
Prior to the Mexican-American war, the boundary between Texas, at that point purportedly an independent Republic, and Mexico was set at the Neuches River by the Treaty of San Jacinto. California, Arizona and New Mexico were at that point still part of Mexico. When President John Tyler signed a joint resolution of Congress admitting Texas to the Union, the resolution set the Texas-Mexican border at the Rio Grande river, some distance south of the Neuches. Tyler signed the resolution a few days before leaving office.
Mexico was angered at the U.S. annexation of Texas and broke diplomatic relations with the U.S. At the same time, the Mexican dictator, Antonio Lopez y Santa Anna repudiated the Treaty of San Jacinto, stating that he had signed it only under duress. The new President, James K. Polk then acted rather surreptitiously. He sent a message to Col. Thomas O. Larkin in California that the U.S. would make no effort to have California join the Union, but
if the people should desire to unite their destiny with ours, they would be received as brethren.
Polk also sent Gen. Zachary Taylor to "guard" the Rio Grande, knowing that the area between the two rivers was a matter of dispute. When eleven U.S. soldiers were killed in a skirmish with Mexican troops, Polk sent a war message to Congress declaring that "American blood has been shed on American soil." While Congress debated the War resolution, Abraham Lincoln, then a Whig Representative from Illinois, offered several "spot resolutions," asking the President to name the spot on American soil where American blood had been shed. Lincoln's resolutions were futile, and the U.S. declared war on Mexico. The war was brief, at the end of which by the treaty of Guadalupe Hidalgo, California, Arizona and New Mexico became part of the U.S. and Texas' admission to the Union was verified. The border was also verified as the Rio Grande.