Complex sentence: Most of the identified German dead in the cemetery died in 1914 and were from units of IX Corps, which originated from the north of Germany from towns like Kiel, Hamburg and Bremen, and in Schleswig-Holstein.
Split into simpler sentences: Most of the identified German dead in the cemetery were from units of IX Corps and had died in 1914. They were largely from the north of Germany from towns like Kiel, Hamburg and Bremen, and in Schleswig-Holstein.