[3][4] On April 30, following the Thornton Affair, Mexican General Mariano Arista's troops began to cross the Rio Grande.

[5] After having dispatched Torrejon, Arista marched with the remainder of the troops and twelve pieces of artillery to Longoreño about five leagues east down the river.

Later Taylor received intelligence that Mexican forces were preparing to cross the river below his position and not believing that Arista would make a direct assault on his fortified camp, concluded that Point Isabel was the true target.

He then gave orders for the batteries at Matamoros to fire upon Fort Brown and sent Pedro de Ampudia with four guns to besiege it.

[8] Taylor now determined to go to the relief of the fort with supplies of ordnance and provisions and on the 7th now set off again for Matamoros with two thousand three hundred men.

[10] Facing south and moving right to left, Taylor, with a force of 2,300 men and 400 wagons,[11] placed Col. David E. Twiggs with Lt. Col. James S. McIntosh's 5th Infantry and Maj. Samuel Ringgold's artillery battery, followed by Capt.

George W. Allen's 4th Infantry, Lt. Thomas Childs' artillery battalion, Lt. Col. William G. Belknap's wing, James Duncan's battery, then Capt.

[13] Taylor halted his columns and formed a line behind his batteries when the Mexican artillery started firing at 2 pm central daylight time.

Arista ordered Torrejon's cavalry to attack the American right, but progress was slow, allowing Twiggs to form the 5th Infantry into a square to meet them with a couple of volleys.

[12] A fire started from a cannon burning wad which halted fighting for an hour as the smoke paralleled between the lines of the opposing forces.

[14] Arista pulled back 1,000 yards on his left and Taylor advanced accordingly, rotating the axis of the battle 40 degrees counterclockwise.

Duncan's battery stopped Arista from turning the American left and then advanced with the 8th Infantry and Ker's dragoons to drive the Mexican right from the field.

Gen. Zachary Taylor 1st Brigade "Left Wing" – Lt. Col. William G. Belknap 2nd Brigade "Right Wing" – Colonel David E. Twiggs Zachary Taylor established Fort Polk,[b] near Point Isabel, 23 miles northeast of present day Brownsville, with a Gulf of Mexico pass suitable for ships' landings, on March 24, 1846, as a supply base for his operations leading up to the Battle of Palo Alto, and used it until 1850.