Complex sentence: Scholars in the 19th and 20th centuries had relied upon Herodotus's account, but Sir Henry Layard's discoveries in the royal archives at Nineveh and Calah have enabled the study of new source material that is several centuries earlier than Herodotus's history.
Split into simpler sentences: Scholars in the 19th and 20th centuries had relied upon Herodotus's account. But, Sir Henry Layard's discoveries in the royal archives at Nineveh and Calah have enabled the study of new source material that is several centuries earlier than Herodotus's history.