War broke out in July 1914. The decision by the party leadership, citing "defense of the fatherland", to implement what amounted to a parliamentary truce ("Burgfriedenspolitik") on votes involving funding for the war was contentious within the party from the outset. An antiwar group quickly emerged in the Württemberg SPD with Clara Zetkin, Fritz Westmeyer and the Thalheimer siblings at its heart. They quickly linked up with the party's pacifist group at a national level, becoming known as the International Group, and headed up by Karl Liebknecht and their friend Rosa Luxemburg. In 1915 Bertha Thalheimer was a co-founder of the anti-war Spartacus League, and was one of the organisers of its launch conference held in Berlin in January 1916. In September 1915, together with Ernst Meyer, she represented the league at the Zimmerwald Conference, an international conference of socialist pacifists held near Bern in Switzerland. She also represented the Spartacus League six months later at the follow-up Kienthal Conference. The conferences demanded an immediate peace, without territorial annexations. They called on the German SPD to reject further war funding. During these years Bertha Thalheimer and her sister-in-law Cläre Thalheimer were also working closely with Leo Jogiches. Although the anti-war message gained popular traction, the political authorities in the leading belligerent powers - except in the Russian empire - were untouched by it: in March 1917 Thalheimer, who had been participating in anti-war street protests in defiance of court orders, was arrested for "anti-military activities" ("wegen "antimilitaristischer Tätigkeit""): in October 1917 she was convicted on charges of high treason by a Stuttgart court, which sentenced her to two years in prison. Her co-accused, Cläre (or Klara) Thalheimer, was acquitted. Bertha served more than a year of the sentence in near Halle which had been adapted for use as a jail.