Highly cultured and sensitive, he was forty-two when he succeeded his father as King of Portugal. He was so overwhelmed with grief that butPedro became the power behind the throne. In 1434 at Evora, Duarte propounded the 'Lei Mental', this decreed that grants of land made to the nobility could descend only in the direct male line of the original grantee and should revert to the crown on the failure of such heirs. This law remained in force until its abolition in 1832. 

In 1437 his favourite brother, Ferdinand, begged to be allowed to lead an expedition to Africa against the infidels. A fleet was duly assembled and the King and his brother set sail with the object of taking Tangier. The town was three miles from the coast and the King, with 8,000 soldiers, was cut off by the Moors from retreating to Ceuta and reduced to extremities in three days. The situation was saved only by Ferdinand offering himself as a hostage. The King and his troops were allowed to return to their ships and sail back to Lisbon. Ferdinand, who became known as 'the Constant Prince', never regained his freedom and died at Fez in June 1443. 
Duarte, who was devoted to his family, never recovered from this ignominious episode, and which probably hastened his death. He fell victim to the plague and died at Tomar in September 1438.

Source: Leo van de Pas