Complex sentence: During World War II Christophersen joined the DNSAP and became a member of its "Storm Section" (SA) and he volunteered to join the Waffen SS on the 7 April 1941.
Split into simpler sentences: During World War II, Christophersen joined the DNSAP and became a member of its "Storm Section" (SA). He volunteered to join the Waffen SS on 7 April 1941.