revision preference src: Song military forces then turned north against the Northern Han, which fell to Song forces in 979. Efforts to take the Sixteen Prefectures were unsuccessful and they were incorporated into the Liao state based in Manchuria to the immediate north instead. <SEP> tgt: Song military forces then turned north against the Northern Han, which fell to Song forces in 979. However, efforts to take the Sixteen Prefectures were unsuccessful and they were incorporated into the Liao state based in Manchuria to the immediate north instead.