Complex sentence: He had a key role in introducing Executive Order 12958 which led to an unprecedented effort to declassify millions of pages from the U.S. diplomatic and national security history and also oversaw Clinton's pardons in the last days of his administration.
Split into simpler sentences: He had a key role in introducing Executive Order 12958 which led to efforts to declassify millions of pages from the U.S. diplomatic and national security history. He also oversaw Clinton's pardons in the last days of his administration.