During the Mexican War, he commanded a squadron during Zachary Taylor's expedition, and saw action in the Battles of Palo Alto, Resaca de la Palma, Monterrey, and Buena Vista.

[2][3] He received a civil education, but applied for a commission directly to President Andrew Jackson, who was impressed by his soldierly appearance, bearing, and skill at horsemanship.

[4] On March 8, 1846, after a final attempt to pressure Mexico to settle on a boundary for Texas, Secretary of War William L. Marcy ordered Brigadier General Zachary Taylor to move his army, which included May's dragoon squadron, to the Rio Grande.

Taylor's destination was the river's north bank, directly opposite the Mexican town of Matamoros, which stood at a natural choke-point and controlled access to well-traveled routes to the south.

When Taylor refused to leave the region, Mexican cavalry ambushed a dragoon detachment under Captain Seth B. Thornton on April 25, 1846, which officially commenced hostilities.

[9][10][11] He led his cavalry squadron on the charge and, despite heavy casualties, secured the objective and silenced the guns before being forced to withdraw due to a lack of infantry support.

[8] Colonel David E. Twiggs, the regimental commander, commented that "After the unsurpassed, if not unequalled charge of Captain May's squadron, the enemy was unable to fire a gun.

[12] The Mexican commander, General Mariano Arista, withdrew his forces to Linares, with Taylor in pursuit for sixty miles before returning to Fort Brown for reinforcements.

[13] The heavily fortified city had a 10,000-man garrison under Arista's replacement, General Pedro de Ampudia, but its supply line running south to Saltillo was vulnerable.

[12] On February 20, 1847, May led a reconnaissance force that included an attached company of Texas Rangers under Major Ben McCulloch and artillery section of six-pounder guns under Captain J.M.

During the mission, the advanced element encountered small units of Mexican General José Vicente Miñón's cavalry brigade and spotted a dust cloud to the south, presumably produced by a much larger force.

Miñón's brigade of 1,500 Mexican lancers flanked the American line and assaulted the supply trains guarded by the Kentucky and Arkansas cavalry, and in the process killed former governor Colonel Archibald Yell.

Chamberlain believed May had received unjustified praise for his actions at Resaca de la Palma and referred to him as the "Murat of America" and an "ass in the lion's skin".