|master of the Order of Santiago|
|constable of Portugal|
|Father||John, Constable of Portugal|
|Mother||Isabel of Barcelos|
Diogo was the eldest son of John, Lord of Reguengos de Monsaraz, Infante of Portugal, and of his wife (and niece) Isabella of Barcelos. Through his father, Diogo was the grandson of King John I of Portugal and through his mother, the grandson of Afonso of Barcelos (future first duke of Braganza).
John of Reguengos had been the master of the Order of Santiago since 1418 and Constable of Portugal since 1431. Upon John's premature death in October 1442, the Portuguese regent, Peter of Coimbra decided to appoint John's son Diogo as the new Constable of Portugal. Diogo was elected on 24 January 1443, as 11th Master of the Order of Santiago. Diogo died a few days later, without issue. Peter appointed his own son, also named Peter, to succeed him as constable.
These appointments were contested by Diogo's half-uncle (and grandfather), Afonso, Count of Barcelos. Afonso had married the daughter of the late beatified constable Nuno Álvares Pereira, and believed their son, Afonso of Ourem, had superior rights to succeed as Constable of Portugal. Peter of Coimbra challenged Barcelos and Ourem to provide documentary proof of the legitimacy of this lineage (which they were unable to produce). Peter of Coimbra attempted to mollify them by creating the title of Duke of Braganza, and investing it in Afonso of Barcelos. However, this failed to heal the breach between Peter and Afonso, which would only grow with time.
|Ancestors of Diogo, Constable of Portugal|
- de Sousa, Antonio Caetano (1735). Historia genealogica da casa real portugueza [Genealogical History of the Royal House of Portugal] (in Portuguese). 2. Lisboa Occidental. p. 167.
"Nobreza de Portugal e do Brasil" – Vol. I, page 297. Published by Zairol Lda., Lisbon 1989.
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