He was one of six members of the Council of the People's Deputies (Rat der Volksbeauftragten) created on 10 November 1918 in Berlin to govern Germany after Kaiser Wilhelm II had abdicated and the Republic had been proclaimed by Karl Liebknecht and Philipp Scheidemann.
[2]: 215 While the former two USPD commissioners were moderate and interested in conciliation with the MSPD, Barth was the most left-wing, associated with Karl Liebknecht, who refused to serve on the Council because it had a non-revolutionary majority.
[2]: 226 That same day, 10 November, Barth first acceded to Ebert's plan to place the revolutionary soldiers back under the command of their (counter-revolutionary) officers, but then changed his position in a drawn-out but stormy speech later that evening.
On 29 December 1918, Barth and the other USPD members resigned from the Council to protest Ebert's use of the military to quash an uprising by revolutionary sailors demanding back pay.
[3] Although he became somewhat more moderate by the end of 1918, Barth had always been the Council's most radical member, calling on workers, for instance, not to 'debase the revolution to a movement for wages,' since that would merely ameliorate conditions, making fundamental change less likely (in an article in Die Rote Fahne, 28 November 1918).