Ferdinand, the ninth King of Portugal and the Algarve, the second but eldest surviving son of Peter I and his wife, Constance of Castile, had succeeded his father in 1367; he is sometimes referred to as the Handsome or rarely as the Inconstant.

On the death of Peter of Castile in 1369, Ferdinand, as great grandson of Sancho IV by the female line, had laid claim to the vacant throne, for which the kings of Aragon and Navarre, and afterwards John of Gaunt, duke of Lancaster (married in 1370 to Constance, the eldest daughter of Peter), had also become competitors.

Meanwhile Henry of Trastamara, the brother (illegitimate) and conqueror of Peter, had assumed the crown and taken the field.

After one or two indecisive campaigns, all parties were ready to accept the mediation of Pope Gregory XI.

The conditions of the treaty, ratified in 1371, had included a marriage between Ferdinand and Leonora of Castile but before the union could take place the former had become passionately attached to Leonor Telles de Menezes, the wife of one of his own courtiers, and having procured a dissolution of her previous marriage, had lost no time in making her his queen.

This strange conduct, although it raised a serious insurrection in Portugal, did not at once result in a war with Henry; but the outward concord had soon been disturbed by the intrigues of the duke of Lancaster, who had prevailed on Ferdinand to enter into a secret treaty for the expulsion of Henry from his throne.

The ensuing war has been unsuccessful, despite English help; and peace is again made in 1373.