started to cooperate with the Czechoslovak National Council, whose objective was the dissolution of Austria-Hungary and the creation of a new state for Czechs and Slovaks. Between 1917 and 1918 Osuský worked as the director of a Czecho-Slovak press agency in Geneva. Between 8–10 April 1918 in Rome he was a representative, together with Milan Rastislav Štefánik, of Slovaks at the Congress of Oppressed Nations. In 1918 he helped organize the Czechoslovak Legions in Italy, established in spite of international treaties prohibiting the creation of military units out of captured enemies. After the creation of Czechoslovakia he began working in the diplomatic services of the new republic. From October 1918 he held the post of the country's diplomatic representative in the United Kingdom. As secretary general of a Czechoslovak delegation in 1919-1920 he participated in the Paris Peace Conference. On 4 June 1920 in Grand Trianon as the extraordinary and plenipotentiary envoy of Czechoslovakia he signed the Treaty of Trianon with Hungary. He later recalled: "I will never forget about one thing. When I was signing my name underneath the treaty carrying the name Trianon, at quarter to five in June 1920, I was fully aware, that I was signing for the final account of the Slovak nation to Kingdom of Hungary, accounting for the costs signed top down by blood, suffering and poverty of my nation. And this statement of account is eternal." He significantly contributed to the functioning of the newly formed League of Nations; between 1921 and 1932 he worked with the Reparations Commission, representing not only Czechoslovakia but also Poland, Yugoslavia, Romania and Greece. From 1922 he was the chairman of the Supervisory Commission - a function he held for fourteen years. He was also active in the Delimitation Commission, tasked with setting new borders in Europe. From 1921 Osuský worked as the Czechoslovak ambassador in France, contributing significantly to the development of Czechoslovak-French relations, a cornerstone of ČSR's international security. He remained at this post until the Second World War. After the breakup of Czechoslovakia in March 1939, he refused to relinquish the embassy and kept the office functioning. He began to organize Czechoslovak foreign resistance. 2 October 1939 Osuský signed an Agreement on the Reestablishment of the Czechoslovak Army in France with French Prime Minister Édouard Daladier. His activity, however, met with resistance, especially with Eduard Beneš. Although in November 1939 he became a