Complex sentence: Between 1957 and 1989, Mielke headed East Germany's Ministry for State Security (MfS or Stasi) and, according to John Koehler, "the longest serving secret police chief in the Soviet Bloc," Koehler (1999), page 72.
Split into simpler sentences: Between 1957 and 1989, Mielke headed East Germany's Ministry for State Security (MfS or Stasi). During his tenure as, "the longest serving secret police chief in the Soviet Bloc," Koehler (1999), page 72.