Complex sentence: On the day Stauffenberg planted his abortive bomb, Graf Hans-Jürgen was on duty at OKH in the Bendlerstrasse and it was his task thus to control the messages which were to mobilise Operation Valkyrie in favour of the coup.
Split into simpler sentences: On the day Stauffenberg planted his abortive bomb, Graf Hans-Jürgen was on duty at OKH in the Bendlerstrasse. It was his task to control the messages which were to mobilise Operation Valkyrie in favour of the coup.