Arriaga completed a degree in mathematics and engineering at the
University of Porto and then volunteered for the
Portuguese Army on 1 November 1935. Taking a military and civil engineering course in the
Military Academy which he graduated from in 1939, he was later assigned to the general staff of the Portuguese Institute of Military Studies. Here he petitioned for reforms to the
conscription system, as well as training and the integration of
paratroopers into the
Portuguese Air Force. Arriaga commanded, as the
Commander in Chief of the Armed Forces, the Portuguese forces in
Mozambique from 1969 until 1974, taking over from General
António Augusto dos Santos and organizing
Operação Nó Górdio ("Operation Gordian Knot") in 1970. This operation, the largest and most expensive military operation performed by the
Portuguese Armed Forces during the entire
Portuguese Colonial War (1961–1974), aimed to attack
FRELIMO in its Northern Mozambican support base, but only led to increased support for the rebellion among the populace. Arriaga was a major political figure in the
Estado Novo regime before the
Carnation Revolution of 25 April 1974 in
Lisbon, holding a number of public positions such as Head of the Ministry of Defense Cabinet, Secretary of State for Aeronautics, Professor of the Institute of High Military Studies, President of the Nuclear Energy Joint Commission and Executive President of the oil company
Angol SA. In March 1974, he organised an abortive
coup d'état against Prime Minister
Marcelo Caetano in an attempt to move the regime to the
right and ensure a
hard line on Portugal's
colonial empire. Following the Carnation Revolution, he was arrested on 28 September 1974 and spent sixteen months in prison. In 1977 he founded the
Independent Movement for the National Reconstruction (MIRN), a right-wing political organisation which appealed to
ultranationalist youth and contested the
1980 Portuguese legislative election as Party of the Portuguese Right (PDP) in coalition with the
Christian Democratic Party and the National Front, receiving 0.4% of the vote. He was the movement's president and then the party's chairman until its extinction in 1984. He died from
Alzheimer's disease in 2004, in Lisbon. ==Decorations==