The Battle of São Mamede marked a crucial turning point in the struggle for power within the nascent County of Portugal. Set in the verdant landscape near Guimarães, the battle unfolded as Afonso I, also known as Afonso Henriques, faced off against the combined forces of his mother, Teresa of León, and her lover, Fernando Pérez de Traba. This conflict was not merely a fight for land; it was a battle for legitimacy and autonomy as Afonso sought to establish himself as the sole ruler of Portugal, in contrast to the influences of the Leonese crown, represented by Teresa.

The lead-up to the battle involved intricate political dynamics. Afonso, following a series of power struggles with his mother, rallied local nobles and soldiers to his cause, dropping the pretenses of allegiance to Leonese authority. The motivations behind this conflict were deeply personal, as Afonso aimed to reclaim his autonomy from Teresa, who had been granted regency over the County by the Leonese kings after the death of Afonso's father. The confrontation was thus characterized by family betrayal as much as by military strategy.

On the day of the battle, Afonso's forces engaged in a fierce confrontation against the assembly led by Teresa and her supporters. Utilizing tactical knowledge of the local terrain and rallying support from his loyalists, Afonso achieved a surprising victory that echoed through the hills of Guimarães. The defeat of Teresa not only cemented Afonso’s power but also marked the beginning of a new era, whereby he would go on to declare the independence of Portugal and later establish the foundations for Portuguese sovereignty and identity.