In 1949, a few years after his father and brothers made the same move, he left with his mother to Northern Germany, where until he was 16 he lived with his family in a refugee camp in Hamburg-Billbrook.
He worked concurrently as a research assistant, and he completed his Ph.D. in 1975; his dissertation was entitled "Zur inneren Gliederung und Verbreitung der vorrömischen Eisenzeit im Niederelbegebiet" (On the structure and dissemination of the pre-Roman Iron Age in the Lower Elbe region).
[2] On 15 April 1975 Häßler took up the position of curator in the then Department of Prehistory in the Lower Saxony State Museum, succeeding Albert Genrich (de).
Häßler was particularly interested in the pre-Roman Iron Age and in the history and ethnogenesis of the Saxons, founding and editing the journal Studien zur Sachsenforschung and excavating and researching graves from Lower Saxony.
These studies followed on the heels of work done by Genrich, who was also a co-founder, and later head, of the Internationales Sachsensymposion, considered the authoritative forum for the discussion of the archaeology of northwestern Europe from the first millennium AD.