Stockholm Peace Association Sandstedt was a central figure in the
Stockholm Peace Association (Stockholms fredsförening), founded in 1885 as a local branch of the national Swedish Peace and Arbitration Society and still active today. Sandstedt ran much of its daily work from his office in downtown Stockholm, managing correspondence, membership records, and the distribution of peace literature. He served as chairman from 1896 to 1923 and later as honorary chairman. In 1925 he edited the book
Minnesskrift vid Stockholms fredsförenings fyratioårsfest (Commemorative Volume for the Stockholm Peace Association's Fortieth Anniversary).
Swedish Peace and Arbitration Society Sandstedt became a principal leader of the
Swedish Peace and Arbitration Society (Svenska Freds- och Skiljedomsföreningen), the world's oldest still-active peace organization, founded in 1883 by Nobel Peace Prize laureate
Klas Pontus Arnoldson. He served the Society for approximately three decades in multiple national offices, including treasurer, secretary, vice chairman, and acting chairman in its central board. From around 1908 he was identified alongside the teacher
Carl Sundblad as one of the Society's two principal leaders. He was made an honorary life member of the Society. Sandstedt co-edited the Society's journal
Fredsfanan (The Peace Flag) with Carl Sundblad. He organized numerous peace congresses within Sweden between 1902 and 1922 and represented the Society at international peace congresses across Europe, including in Stockholm (1910),
Geneva (1912),
The Hague (1913),
Basel (1921),
London (1922),
Basel (1923),
Berlin (1924),
Paris (1925), and
Geneva (1926).
Peace Monument Bureau From 1913, Sandstedt served as secretary of the Peace Monument Bureau (Fredsmonumentets byrå), responsible for the fundraising and administration of the peace monument on the border between
Eda Municipality (
Sweden) and
Magnor (
Norway) in the
transborder park now called
Morokulien. The bureau was established at the Swedish Peace and Arbitration Society's offices by a joint working committee whose members included Carl Sundblad, Emil Larsson,
J. L. Saxon, the sculptor
Carl Eldh, and the women's suffrage leader
Ann Margret Holmgren. It was erected jointly by the Swedish Peace and Arbitration Society and to mark the centenary of peaceful relations between Sweden and Norway, and was inaugurated on 16 August 1914.
Northern Peace Federation Sandstedt was a co‑founder of the
Northern Peace Federation (Nordiska fredsförbundet), established in 1918 to coordinate peace work across Sweden, Norway, and
Denmark in the aftermath of
World War I. The federation functioned as a Nordic umbrella organization, with one national branch in each country. Sandstedt served as General Secretary of its Northern Peace Congress, which convened a Nordic peace congress in
Copenhagen on 1–2 July 1921.
Universal language In 1920, Sandstedt conducted an international survey on behalf of the Northern Peace Federation, distributing a questionnaire to
philologists at universities across several countries to determine which language — living or dead — would best serve as a shared medium for international correspondence, in support of the Federation's mission of fostering peace among nations. Of the 34 responses received, 26 favoured
English and rejected constructed languages such as
Esperanto,
Ido, and
Volapük; two proposed
French, one
German, one
Latin, one Esperanto, and three Ido. Sandstedt endorsed the majority view, citing the 1907 pamphlet
Zur Kritik der künstlichen Weltsprachen by
Karl Brugmann and
August Leskien, which argued that constructed languages, lacking native speakers and any literary tradition, could not match the organic richness of a living tongue already in worldwide use. The resolution he transmitted from the Northern Peace Congress in Copenhagen, held 1–2 July 1921, expressed the congress's view that English should be adopted as the world's international language on equal footing with each nation's own — with particular emphasis on winning over the smaller nations — while separately urging governments and the
League of Nations to commission an independent inquiry, its findings to form the basis for introducing the recommended language into schools and colleges worldwide. Sandstedt and Emil Larsson continued to study the question for the Federation from its headquarters in
Stocksund for several years thereafter.
Swedish World Peace Mission In his later years Sandstedt was involved in more radical
pacifism. He served as vice chairman of the
Swedish World Peace Mission (Svenska Världsfredsmissionen), a Christian peace organization founded in 1919 when 875 members broke away from the Swedish Peace and Arbitration Society to focus on
conscientious objection and opposition to Swedish arms exports. By 1932 the organization numbered approximately 4,500 members, affiliated with the
War Resisters' International, and operated a "liberation fund" (
frigörelsefonden) intended to help workers leave the
armaments industry; similar funds were reported that year in Switzerland and France. == Animal welfare ==