Complex sentence: After he left the Detachment he took a sabbatical for a year before returning to the SBS where he was posted to MAT ("Maritime Anti-Terrorism") operations in which he was involved in a number of exercises retaking oil rigs and tankers.
Split into simpler sentences: After he left the Detachment he took a sabbatical for a year before returning to the SBS. After returning to SBS he was posted to MAT ("Maritime Anti-Terrorism") operations in which he was involved in a number of exercises retaking oil rigs and tankers.