Theresa, Countess of Portugal in the context of "Urraca of León and Castile"

The Battle of São Mamede (Portuguese: Batalha de São Mamede, pronounced [ˈsɐ̃w mɐˈmɛðɨ]) took place on 24 June 1128 near Guimarães and is considered the seminal event for the foundation of the Kingdom of Portugal and the battle that ensured Portugal's independence. Portuguese forces led by Afonso Henriques defeated forces led by his mother Teresa of Portugal and her lover Fernão Peres de Trava. Following São Mamede, the future king styled himself "Prince of Portugal". He would be called "King of Portugal" starting in 1139 and was recognised as such by neighbouring kingdoms in 1143.

Dom Afonso I (born Afonso Henriques; 1106/1109/1111 – 6 December 1185) nicknamed "the Conqueror" (Portuguese: O Conquistador) and "the Founder" (O Fundador) by the Portuguese, was the first king of Portugal, from 26 July 1139 until his death on 6 December 1185. He achieved the independence of the County of Portugal, establishing a new kingdom and doubling its area with the Reconquista, an objective that he pursued until his death.

Afonso was the son of Theresa of León and Henry of Burgundy, rulers of the County of Portugal. Henry died in 1112, leaving Theresa to rule alone. Unhappy with Theresa's romantic relationship with Galician Fernando Pérez de Traba and his political influence, the Portuguese nobility rallied around Afonso, who revolted and defeated his mother at the Battle of São Mamede in 1128 and became sole Count of Portugal soon afterwards. In 1139, Afonso renounced the suzerainty of the Kingdom of León and established the independent Kingdom of Portugal.