His military service took place between 1967 and 1971 in Angola, as a helicopter pilot, with the rank of Lieutenant, where he made many contacts with the local population, and learned local dialects. He was expelled from Angola in 1973, escorted out by the political police, for having challenged the dictatorial regime by helping to organize an opposition for the next elections.
In 1987 he completed the course of the Institute of National Defence and qualified as a Pilot Captain Aviator in the Portuguese Air Force Reserve. He remembers his military services as one of the most fulfilling periods of his life, especially because of his contacts maintained with the local peoples, who always received him very warmly.
But his feeling of love and obligation to the people of the former Portuguese colonies is not confined to Africa. Dom Duarte has taken a keen interest in East Timor, since its annexation by Indonesia, and has been a determined fighter for the freedom of this martyred people. Even before the Portuguese Government took up a strong pro-Timor stance, Dom Duarte was already active in his support. As President of ‘Timor 87’, Dom Duarte promoted a campaign to support Timor and the Timorese residents in Portugal and elsewhere, which has been supported by Maria Cavaco Silva (wife of the then Portuguese Prime Minister), João Soares (son of the then President of the Republic), the Commander in Chief of the Portuguese Air Force and representatives of the two major Portuguese Trade Unions (Intersindical and UGT), which built a block of forty houses for Timorese refugees.
For the past thirty years, one of D. Duarte’s more passionate struggles is the apparently lost fight for the preservation and development of Portuguese agriculture, about which he once said: "Agriculture continues to play a fundamental part in the lives of the nations. Apart from the production of essential food products, it is also the only activity responsible for the humanisation and permanent occupation of the rural areas". He has been an active member of the League for the Protection of Nature and of various other organizations with ecological objectives and working, towards the preservation of natural resources.
Dom Duarte is also a past President of the National Federation of Mutual Agricultural Credit (Federação Nacional de Crédito Mutuo), and a member of the Council of CONFAGRI (the National Confederation of Agricultural Co-operatives); he is President of the Dom Manuel II Foundation, which was founded in 1966 to aid Portuguese emigrants, through help in housing, integration into the local community and teaching (and has over the years sent hundreds of thousands of dollars’ worth of aid to East Timor), while the Portuguese Heritage Foundation, based in North America, has the aim of preserving and disseminating the history and cultural heritage of Portugal throughout the world.