On January 31, 1910, when the PSDR was founded, Frimu, Rakovsky (who had been allowed to return), Gheorghiu Bujor, Dimitrie Marinescu and Constantin Vasilescu, were elected among its leaders. With Rakovsky, Gheorghiu Bujor, and Ecaterina Arbore, he gave lectures at the PSDR's propaganda school. After the outbreak of World War I, Frimu and the PSDR stood among the socialist internationalist opponents of war, and Rakovsky played a major part in the Zimmerwald movement. As Romania entered the war on the side of the Entente Powers, the PSDR was banned and Rakovsky jailed. When the Central Powers occupied southern Romania ("see Romanian Campaign"), Frimu, who was himself jailed for his anti-war propaganda, followed the authorities into Moldavia. In September 1917, I. C. Frimu and Alecu Constantinescu were the Romanian delegates to the third internationalists' conference against war in Stockholm. In late December 1918, he joined the Socialist Party of Romania, created around the PSDR. On December 6, 1918, typesetters from the Sfetea and Minerva graphic design studios went on strike demanding better working and living conditions (a pay raise, an eight-hour day, etc.). As their demands were not met, all the typographers in Bucharest announced that they would strike a week later. On December 13, 1918 (NS December 26), a large socialist demonstration of Bucharest workers took place. Its principal organisers were Rakovsky, the typographers Iancu Luchwig and Sami Steinberg, the bootmaker Marcus Iancu, the proofreader Marcel Blumenfeld, Ilie Moscovici, Frimu, Gheorghe Cristescu, D. Pop and others. That day, the capital's nearly 600 typesetters went on strike and moved toward the Ministry of Industry and Commerce, accompanying the delegation which they had designated to present their grievances. Workers from other Bucharest factories and plants joined the typographers in solidarity. The protesters shouted slogans such as "Down with the army! Down with the King! Long live the Republic!", and expected to end up before the National Theatre. As soon as the columns of workers reached the square in front of the National Theatre, they were met by forces of public order, specifically the 9th "Vânători de Munte" Regiment, the police and the gendarmerie, commanded by police prefect General Ştefănescu and by the head of the Bucharest garrison, General Mărgineanu.