After the United States announced the annexation of Texas in 1845, a dispute over who owned Texas and where its border lay deepened into war. President James Polk proclaimed the Rio Grande the south 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. Diplomacy failed, both presidents ordered armies to march, and on May 9th, 1846, the second battle of the U.S.-Mexican War erupted here in the dense thickets along a ravine called Resaca de la Palma. At stake was the fate of the lands north of the river. The war first broke out 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.