In 1898, war broke out between the United States and Spain.
O'Neill joined the Rough Riders and became Captain of Troop A. First Lieutenant Frank Frantz served as O'Neil's Deputy Commander.
Along with Alexander Brodie and James McClintock, he tried to make an entire regiment made up of Arizona Cowboys.
Eventually though, only three troops were authorized.

The Rough Riders landed at Daiquirí on June 22, 1898.
Two Buffalo Soldiers, of the 10th Cavalry fell overboard.
Upon seeing this, O'Neill jumped into the water in full uniform and sabre.
He searched for the men for two minutes, before having to come up for breath.

On June 25, 1898, the Rough Riders saw their first action.
O'Neill led his men at the front of the line in the Battle of Las Guasimas, capturing the Spanish flank.
During the action he saw several men, who he believed were Spaniards, across the road from him, and shouted "Hostiles on our right, fire at will!"
He learned after the firing ceased that the men he exchanged shots with were Cuban rebels.
In 1898, war broke out between the United States and Spain. O'Neill joined the Rough Riders and became Captain of Troop A. First Lieutenant Frank Frantz served as O'Neil's Deputy Commander. Along with Alexander Brodie and James McClintock, he tried to make an entire regiment made up of Arizona Cowboys. Eventually though, only three troops were authorized.
The Rough Riders landed at Daiquirí on June 22, 1898. Two Buffalo Soldiers, of the 10th Cavalry fell overboard. Upon seeing this, O'Neill jumped into the water in full uniform and sabre. He searched for the men for two minutes, before having to come up for breath.
On June 25, 1898, the Rough Riders saw their first action. O'Neill led his men at the front of the line in the Battle of Las Guasimas, capturing the Spanish flank. During the action he saw several men, who he believed were Spaniards, across the road from him, and shouted "Hostiles on our right, fire at will!" He learned after the firing ceased that the men he exchanged shots with were Cuban rebels.
Buckey O'Neill