In 1946 he became director at N.V. De Sleutels, like his grandfather. In 1964, Keys was acquired by
Meneba, and for three years de Koster was a board member. In the
De Jong cabinet (1967-1971), he was
State Secretary for Foreign Affairs on behalf of the
People's Party for Freedom and Democracy (Dutch:
Volkspartij voor Vrijheid en Democratie, VVD). He was subsequently
defence minister in two Biesheuvel cabinets (
1971-1972 and
1972-1973). In 1972 de Koster presented the Rijckevorsel Commission report which proposed changes in the
Dutch armed forces, including a major reduction in the army and transfer of Air Force roles to
NATO partners, build a training area at
Ter Apel and the first step towards a volunteer professional army. After his time as minister, he was a member of the
Dutch House of Representatives and spokesman on Foreign Affairs of the VVD (1973-1977) and from 1977 to 1980 he was a member of the
Dutch Senate. From 1978 to 1981 he was Chairman of the Consultative Assembly of the
Council of Europe. From his personal archives, released in 2005, it was revealed that he breached the confidentiality of the private fixed Parliamentary Committee for Defence in June 1975, and the prince was informed about the developments. He also knew through a filibuster during a meeting of the committee to avoid research into the prince. ==Decorations==