Three years after the death of the King Bayinnaung, Naresuan’s father declared that he was no longer a vassal of the Burmese. During the ensuing war Crown Prince Naresuan led his father’s forces as well as those he recruited from neighbouring regions. Rather than fight a defensive campaign he decided to take the fight to the enemy occupying Lanna, a buffer states between the two countries.
In 1590, Naresuan’s father died and he became King Sanphet II. One year later the Burmese invaded and again Naresuan went on the offensive in spite of the numerical superiority of the Burmese forces. On 18th January 1591, along with his younger brother, Prince Ekatosrost, he led a small force in an attempt to lure the opposing army into a trap. The Burmese forces took the bait, chasing Naresuan’s small force into an ambush at Nongsarai.
During the battle, Naresuan spotted the Burmese Crown Prince Minchit Sra, who he challenged to single combat. The two fought atop their war elephants until Naresuan killed Minchit Sra with a blow from his lance. So demoralised were the Burmese that they retreated with the body of the Prince, effectively ending the Burmese aggression against Ayutthaya.
The date of the battle is celebrated today in Thailand as Royal Thai Armed Forces Day.