Ledebour was a member of the German Reichstag from 1900 until 1918. He took part in the international anti-war socialist conferences at Zimmerwald in 1915, and in Stockholm in 1917. He was one of the leaders of the German Independent Social Democratic Party (USPD) after the split in the SPD in 1917. The Majority Social Democratic Party of Germany (MSPD) broadly supported the German government's war aims, and the USPD was opposed to the government.