Reference: Al-Ajwibah al-Jaabiriyyah ‘alaa As.ilah al-Bareediyyah al-Iliktroniyyah – Question 6, Saturday 20 Rajab 1428AH
http://www.sahab.net/forums/showthread.php?t=370694
Question: Is it permissible to study the laws of the disbelievers in order to defend the Muslims who are being oppressed in this land [of the disbelievers]?
Response: If the intent in studying the laws of the disbelievers is to defend the Muslims who are resident in the land of the disbelievers, then there is no harm in that, in shaa.-Allaah.