Complex sentence: The Firearms Wing, as it was then called, was formally established as part of the Civil Defence and Communications Branch (D6) in December 1966, in response to the murder of three officers in the Massacre of Braybrook Street.
Split into simpler sentences: The Firearms Wing, as called when it was established, was formed as part of the Civil Defence and Communications Branch (D6) in December 1966. Its creation being in response to the murder of three officers in the Massacre of Braybrook Street.