Complex sentence: More recently, the Archive uncovered a secret reclassification program operating unabated since 1999 which was an effort to reclassify many holdings related to American foreign policy during the 1940's and 1950's at the National Archives and Records Administration.
Split into simpler sentences: More recently, the Archive uncovered a secret reclassification program operating since 1999. This program was underway to reclassify documents related to American foreign policy during the 1940's and 1950's, at the National Archives and Records Administration.