During the Second World War he served as a soldier in Russia, but was dismissed from military service as incurably ill due to a severe dysentery.
[2] He spent the end of the war in 1944/45 in Tengen at Lake Constance.
In 1946 he was assigned lecturer of music history at the Musikhochschule Freiburg, a position he held until 1958.
In 1954 his habilitation qualified him as private lecturer of musicology at the University of Freiburg im Breisgau, where he was appointed extraordinary professor in 1962.
In 1963, Hammerstein accepted the call for a full professorship in musicology at the Ruprecht-Karls-Universität Heidelberg, which he held until his [retirement] in 1980 Hammerstein, who expanded the field of musicology to include iconography, married Dr. Irmgard Hueck, born in 1943, with whom he had three children.