[11] He threw down calls from socialists in USA to mediate an end of the war with the German government, while defending the resumption of submarine warfare by German Kriegsmarine as response to the rejection of "Germany's sincere offer of immediate peace negotiations"[12] On 15 November 1918 he signed the Stinnes-Legien Agreement with industrialist Hugo Stinnes, an agreement in which the German employers for the first time accepted nationwide unions as legitimate workers-organisations and which introduced an eight-hour day, workers councils in plants with more than 50 employees and parity employment offices.

To Legien the expected loss of Upper Silesia to Poland after World War I would intensify the 'psychological' impact of the "draconian peace" on the German working class.

[16] He countered the right-wing Kapp Putsch of March 1920 by organizing a massive general strike in Germany[17] with about 12 million employees following the joint call of the legal government and the unions.

[18][19] The strike immediately halted all production, transportation, mining and public services, it was "the strongest mass movement the German proletariat ever created"[4] and "gave the Kapp régime its death blow".

[5][20] Legien died after a short illness in Berlin and was buried at Zentralfriedhof Friedrichsfelde,[19][21] where his grave now forms part of the Memorial to the Socialists (German: Gedenkstätte der Sozialisten).