After the United States moved to annex Texas in 1845, President James Polk declared the Rio Grande the southern boundary of Texas, while Mexican President Mariano Paredes insisted that Mexican territory extended hundreds of miles north of the river and refused to accept that Texas was permanently lost. When diplomacy failed, both presidents sent armies forward. On May 9, 1846, the second battle of the U.S.-Mexican War erupted here in the dense thickets along the ravine called Resaca de la Palma. The fate of the lands north of the river was at stake. The war first broke out here in the Rio Grande valley, then spread west to California and south to Mexico City, and in the end the northern half of Mexico became the southwestern third of the United States.