Three months after the outbreak of the rash of revolutionary outbreaks that swept across the cities and towns of Germany following the abdication, Katz became a in February 1919. Unlike many of those who would come to prominence as leading figures of the political extreme left in Germany during the 1920s, Katz had not at this stage switched his party allegiance to the anti-war Independent Social Democratic Party. It was as a member of the SPD that in June 1919 he attended the party's at Weimar, a small town which had acquired a hitherto implausible degree of political significance earlier in the year.