On the outbreak of the First World War, the SDP leader, Friedrich Ebert, ordered members in the Reichstag to support the war effort. Karl Liebknecht was the only member of parliament who voted against Germany's participation in the war. He argued: "This war, which none of the peoples involved desired, was not started for the benefit of the German or of any other people. It is an Imperialist war, a war for capitalist domination of the world markets and for the political domination of the important countries in the interest of industrial and financial capitalism. Arising out of the armament race, it is a preventative war provoked by the German and Austrian war parties in the obscurity of semi-absolutism and of secret diplomacy."
In April 1917 left-wing members of the Social Democratic Party formed the Independent Socialist Party (USPD). Members included Barth, Kurt Eisner, Karl Kautsky, Rudolf Breitscheild, Julius Leber, Ernst Thälmann, Emil Eichhorn and Rudolf Hilferding.
On 28th October, 1918, Admiral Franz von Hipper and Admiral Reinhardt Scheer, planned to dispatch the fleet for a last battle against the British Navy in the English Channel. Navy soldiers based in Wilhelmshaven, refused to board their ships. The next day the rebellion spread to Kiel when sailors refused to obey orders. The sailors in the German Navy mutinied and set up councils based on the soviets in Russia. By 6th November the revolution had spread to the Western Front and all major cities and ports in Germany.
On 9th November, 1918, Kaiser Wilhelm II abdicated and the Chancellor, Max von Baden, handed power over to Friedrich Ebert, the leader of the German Social Democrat Party. At a public meeting, one of Ebert's most loyal supporters, Philipp Scheidemann, finished his speech with the words: "Long live the German Republic!" He was immediately attacked by Ebert, who was still a strong believer in the monarchy and was keen for one of the his grandsons to replace Wilhelm. Barth became one of six members of the Council of the People's Deputies that was created on 10th November in Berlin.
On 29th December, 1918, Ebert gave permission for the publishing of a Social Democratic Party leaflet that attacked the activities of the Spartacus League, led by Karl Liebknecht, Rosa Luxemburg, Leo Jogiches and Clara Zetkin: "The shameless doings of Karl Liebknecht and Rosa Luxemburg besmirch the revolution and endanger all its achievements. The masses cannot afford to wait a minute longer and quietly look on while these brutes and their hangers-on cripple the activity of the republican authorities, incite the people deeper and deeper into a civil war, and strangle the right of free speech with their dirty hands. With lies, slander, and violence they want to tear down everything that dares to stand in their way. With an insolence exceeding all bounds they act as though they were masters of Berlin." Barth and the other USPD members resigned from the Council to protest Ebert's behaviour.
On 4th January, 1919, Friedrich Ebert, Germany's new chancellor, ordered the removal of Emil Eichhorn, as head of the Police Department in Berlin. Chris Harman, the author of The Lost Revolution (1982), has argued: "The Berlin workers greeted the news that Eichhorn had been dismissed with a huge wave of anger. They felt he was being dismissed for siding with them against the attacks of right wing officers and employers. Eichhorn responded by refusing to vacate police headquarters. He insisted that he had been appointed by the Berlin working class and could only be removed by them. He would accept a decision of the Berlin Executive of the Workers' and Soldiers' Councils, but no other."
Members of the Independent Socialist Party and the German Communist Party jointly called for a protest demonstration. They were joined by members of the Social Democratic Party who were outraged by the decision of their government to remove a trusted socialist. Eichhorn remained at his post under the protection of armed workers who took up quarters in the building. A leaflet was distributed which spelt out what was at stake: "The Ebert-Scheidemann government intends, not only to get rid of the last representative of the revolutionary Berlin workers, but to establish a regime of coercion against the revolutionary workers. The blow which is aimed at the Berlin police chief will affect the whole German proletariat and the revolution."
One of the organisers of the protests, Paul Levi, argued: "The members of the leadership were unanimous: a government of the proletariat would not last more than a fortnight... It was necessary to avoid all slogans that might lead to the overthrow of the government at this point. Our slogan had to be precise in the following sense: lifting of the dismissal of Eichhorn, disarming of the counter-revolutionary troops, arming of the proletariat. None of these slogans implied an overthrow of the government."
Friedrich Ebert, Germany's new chancellor, called in the German Army and the Freikorps to bring an end to the rebellion. By 13th January, 1919 the rebellion had been crushed and most of its leaders were arrested. This included Rosa Luxemburg, Karl Liebknecht and Wilhelm Pieck on 16th January. Paul Frölich, the author of Rosa Luxemburg: Her Life and Work (1940) has explained what happened next: "A short while after Liebknecht had been taken away, Rosa Luxemburg was led out of the hotel by a First Lieutenant Vogel. Awaiting her before the door was Runge, who had received an order from First Lieutenants Vogel and Pflugk-Hartung to strike her to the ground. With two blows of his rifle-butt he smashed her skull. Her almost lifeless body was flung into a waiting car, and several officers jumped in. One of them struck Rosa on the head with a revolver-butt, and First Lieutenant Vogel finished her off with a shot in the head. The corpse was then driven to the Tiergarten and, on Vogel's orders, thrown from the Liechtenstein Bridge into the Landwehr Canal, where it was not washed up until 31 May 1919."
Paul Frölich, the author of Rosa Luxemburg: Her Life and Work (1940) has explained what happened: "A short while after Liebknecht had been taken away, Rosa Luxemburg was led out of the hotel by a First Lieutenant Vogel. Awaiting her before the door was Runge, who had received an order from First Lieutenants Vogel and Pflugk-Hartung to strike her to the ground. With two blows of his rifle-butt he smashed her skull. Her almost lifeless body was flung into a waiting car, and several officers jumped in. One of them struck Rosa on the head with a revolver-butt, and First Lieutenant Vogel finished her off with a shot in the head. The corpse was then driven to the Tiergarten and, on Vogel's orders, thrown from the Liechtenstein Bridge into the Landwehr Canal, where it was not washed up until 31 May 1919."
In 1920 Barth published his memoirs as From the Workshop of the Revolution, in which he claimed that the Independent Socialist Party had worked toward fomenting revolution during the First World War. In 1921 he rejoined the Social Democrat Party.
Emil Barth died in Berlin on 17th July, 1941.