American Military History, Volume 1 by Richard W. Stewart

On the evening of May 9, the day of the battle of Resaca de la Palma, President Polk received a message from the War Department that informed him of the attack on Captain Thornton's detachment on April 25. Polk, already convinced by the breakdown in negotiations with Mexico that war was justified, immediately drafted a message declaring that a state of war existed between the United States and Mexico. Congress passed the declaration, and Polk signed it on May 13. Congress then appropriated $10 million and substantially increased the strength of the Army. (After the Second Seminole War the authorized strength had been cut from 12,500 to 8,500. This had been done by reducing the rank and file strength of the regiments, instead of eliminating units, thus firmly establishing the principle of an expansible Army.) Congress raised the authorized enlisted strength of a company from 64 to 100 men, bringing the rank and file up to 15,540, and added a regiment of mounted riflemen and a company of sappers, miners, and pontoniers (engineers for pontoon bridges). Also, the President was authorized to call for 50,000 volunteers for a term of one year or the duration of the war.

The President went into the war with one object clearly in view-to seize all of Mexico north of the Rio Grande and the Gila River and westward to the Pacific. After his discussions with Maj. Gen. Winfield Scott, the outlines of a three-pronged thrust emerged. (Map 20) General Taylor was to advance westward from Matamoros to the city of Monterrey, the key to further progress in northern Mexico. A second expedition under Brig. Gen. John E. Wool was to move from San Antonio to the remote city of Chihuahua in the west, an expedition later directed southward to Saltillo near Monterrey. A third prong under Col. Stephen W. Kearny was to start at Fort Leavenworth, capture Santa Fe and ultimately continue to San Diego on the coast of California. Part of Kearny's forces under volunteer Col. Alexander W. Doniphan later marched south through El Paso to Chihuahua. Although small in numbers, only 1,600 strong, Kearny's and Doniphan's forces posed strategic threats to Mexico's northern states.