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, Frimu 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 the 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 to Moldavia.