Complex sentence: Hartmann had resisted the Nazi regime in the 1930s and 1940s and had witnessed the development of other, violent successors, so his use of Giraudoux's text suggests an ongoing folly, that the constructors of empire never learn.
Split into simpler sentences: Hartmann had resisted the Nazi regime in the 1930s and 1940s, and had witnessed the development of other, violent successors. Hence his use of Giraudoux's text suggests an ongoing folly, that the constructors of empire never learn.