War ended in defeat for Germany in May 1945. Halle was now administered as part of the Soviet occupation zone, applying a reconfigured set of internal and external frontiers agreed between the winning powers a few months earlier. Trades unions were no longer banned and in 1945 Horst Heintze joined the recently established &lt;a href="Free%20German%20Trade%20Union%20Federation"&gt;Free German Trades Union Federation (FDGB / "Freier Deutscher Gewerkschaftsbund ")&lt;/a&gt;. Two years later, in 1947, which was the year of his twentieth birthday, he became a member of the newly formed &lt;a href="Socialist%20Unity%20Party%20of%20Germany"&gt;Socialist Unity Party (SED / "Sozialistische Einheitspartei Deutschlands")&lt;/a&gt;. Between 1945 and 1948 he returned to factory work in the metals-based sector, working in a southern quarter of Halle. In 1948/49 he also served as FDGB Secretary for the Halle district union executive. moving on to served between 1949 and 1952 as FDGB regional First Secretary for the whole of Saxony-Anhalt. In the meantime, in October 1949 the Soviet occupation zone had been re-established as the Soviet sponsored German Democratic Republic, a standalone German state with constitutional arrangements closely modeled on those of the Soviet Union itself. In 1952/53 he served as chairman of the FDGB regional leadership for Magdeburg.