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The military dictatorship established by the coup of 28 May 1926, brought Lieutenant Colonel Augusto Botelho da Costa Veiga to the directorship of the National Library where he combined his role in military defence with historical research. His focus was primarily on military history, but he also conducted studies in the field of medieval history, particularly those on the actions of D. Afonso Henriques and the episode of the Battle of Ourique, about which he engaged in some controversy with the ideas defended by Herculano, earning him a place among the experts on the subject. Serving as director of the National Library from 1928 to 1950 facilitated his access to documentation, allowing him to write many of his works, the quality of which earned him recognition as an authority in medieval military history and an invitation to join the Lisbon Academy of Sciences as a full member in 1955, occupying the seat left vacant by António Ferrão, who had become an emeritus member. His activities included the publication of various works, bibliographies, catalogues, and the inventory of the National Library's collections.
During his term of office, the publication of Decree-Law 19.952 (27/6/1931) marked a significant reorganisation of libraries and archives, streamlining services and reducing costs. This decree reshaped the configuration of these institutions but, in practice, reinforced the concept of the National Library as a "heritage library," a "vault," a "reserve," contrasting with the popular notion of free and easy access for all and establishing a clear separation between administrative and technical services. The National Library of Portugal (BNL) was considered the “central and national repository” of the Portuguese Book Collection, centralising Portuguese bibliography and the registration of authors' property rights. Despite the grandeur of its mission, the National Library lacked the logistical conditions to ensure compliance with the mandates set forth by official legislation. This inadequacy was evidenced by photographic records showing some books and documents in the library's collection, partially damaged by insects and haphazardly arranged on disordered shelves—a grim inventory reflecting the institution's neglected state, similar to the situation revealed by Jaime Cortesão upon assuming the directorship of the Library.
The 1950s began with the brief administration of João Martins da Silva Marques. An archivist by profession and historian, he left the position after being appointed to the Faculty of Letters of Lisbon to teach Palaeography and Diplomatics, aligning with his primary inclination, as seen in his choice of successor: former pupil, Manuel Santos Estevens, would lead the BNL until 25 April 1974.
