Santos Costa and Salazar met at the University of Coimbra in 1917, where they were both members of a Catholic student organisation, the Academic Centre for Christian Democracy.
Santos Costa was set to become one of the leading figures of the New State, the style adopted by the Salazar dictatorship, responsible for the reforms intended to improve the fighting efficiency of the Army.
After the outbreak of the Second World War in 1939, he was also strongly associated with the faction in both the State and Army that desired a German victory, even though this went against the good relations that Portugal traditionally enjoyed with England.
Although far more right-wing than Salazar himself, and something of a dangerous political maverick, he was finally appointed full Minister of War in 1944, because of his skill in ensuring that the Army remained an effective prop for the dictatorship.
In August 1958, in a surprise move by Salazar, Santos Costa was finally dropped from the government, along with Marcelo Caetano, from the more liberal wing of the New State apparatus.