In 1548 his father ascended the throne as
King of Ayutthaya, he immediately became heir and
Uparaja of Siam. During the
First Siege of Ayutthaya war with
Toungoo Dynasty, Prince Ramesuan together with the King, the Queen, Prince Mahin and Princess Boromdhilok left the walls of the city on their
war elephants to engage the Burmese forces led by King
Tabinshwehti of
Pegu in battle. In the combat with the
Thado Dhamma Yaza, the Viceroy of
Prome both his mother and his sister lost their lives. It was recorded in Siamese history that it was Prince Ramesuan who returned his mother's lifeless body to the capital. After a failed siege of the
capital city, Tabinshwehti and his forces decided to retreat northward near
Mae Sot. Prince Ramesuan and Maha Thammaracha was ordered to pursue the retreating forces, costing many Burmese lives. Soon the Burmese decided to stand ground and ambush the Siamese forces near
Kamphaeng Phet, by dividing their forces on two sides of the road and outflanking Prince Ramesuan's forces. As a result, Prince Ramesuan and Maha Thammaracha was captured by the Burmese. This prompted Maha Chakkraphat to negotiate a peace with Tabinshwehti, which resulted in the turning over of two great war elephants and a cease fire. Prince Ramesuan and Thammaracha was released and the Burmese was allowed to retreat unmolested. After the war, the Prince was part of the party inside the Royal court that favoured the dismantling of the walls of the cities of
Suphanburi,
Lopburi and
Nakhon Nayok, this was implemented as a way of depriving a future Burmese invasion with a fortified stronghold, only a day's march from the capital. ==Second war==