Neutral at first, by 1916 Romania's government was increasingly open to participation in the war on the Entente side, and decided to crack down on the socialist movement, brutally repressing a pacifist demonstration in Galați in June.
With a significant part of the members drafted (including the general secretary, Dimitrie Marinescu, killed in action), the party was only able to retain a clandestine activity in the main industrial centres of the country, such as Bucharest, Iași and the Prahova Valley.
The Bucharest clandestine group, led by Alecu Constantinescu and Gheorghe Cristescu, emerged as the "maximalist" Central committee for anti-war and anti-imperialist action, also coordinating the "intimate councils" active in Ploiești and Câmpina.
The Bucharest committee also saluted the Revolution and condemned the humiliating peace between Romania and the Central Powers, leading the Germans to reverse their earlier policies and imprison all pre-war socialist leadership in May 1918.
In the meantime, in April 1918, the moderate social-democrats remaining in Iași, led by Litman Ghelerter and Ion Sion, also regrouped the local party sections into the Moldavia Regional Committee.