Complex sentence: Bangkok in the late 1950s was a relatively small, secretive and highly respectable city which was not yet the open-air brothel that it would become during the mid- 1960s and early 1970s.
Split into simpler sentences: Bangkok in the late 1950s was a relatively small, secretive and highly-respectable city. It was not yet the open-air brothel that it would become during the mid- 1960s and early 1970s.