Млада Босна
Gaćinović, who was personal friends with European socialists such as Victor Serge and Julius Martov, met Leon Trotsky by chance in Paris. His revolutionary zeal impressed Trotsky. From autumn 1910 to the summer of 1912, Gaćinović was a student at Vienna University. In his late teens, after visiting the Kingdom of Serbia, Gaćinović organized underground cells, "kruzoks", amongst Serbs in Bosnia and Herzegovina, Zagreb, and western Slavonia. In 1911, he became the only Young Bosnia leader to join Unification or Death, Dragutin Dimitrijević's secret society. In the same year, the term "Mlada Bosna" (Young Bosnia) was popularized in an article by Gaćinović, and modeled by him after Young Italy and Young Russia. In 1912, Gaćinović publicized a letter by one of his associates, with whom he fully agreed. Titled "Cry of a Desperate One" (), it attacked the younger generations, in particular students at foreign universities, for lack of idealism and opportunism, petty individualism, and conformity.