In April 1917, he was a founding member of the Independent Social Democratic Party (USPD), a more leftist and anti-war breakaway from the SPD.

On 5 February 1918, he was found guilty of attempted treason by a military court for his involvement in the Berlin munitions workers' strike and sentenced to five years in prison.

After Germany began to seek a ceasefire to end the war, he was released on 15 October 1918 as part of the change of course in domestic politics under Chancellor Max von Baden.

[2] During the first weeks of the German Revolution of 1918–1919, Dittman was a member for the USPD on the Council of the People's Deputies, the six-man body that functioned as Germany's revolutionary government beginning on 10 November 1918.

On 22 and 23 January 1926, Dittmann gave a six-hour speech to the Reichstag's parliamentary committee of enquiry into the stab-in-the-back myth, which he chaired.

[1] Dittmann's memoirs, written in Switzerland between 1939 and 1947 and published in 1995, are a first-rate autobiographical source on the history of the German labour movement, particularly during the First World War, the November Revolution and the first years of the Weimar Republic.