Complex sentence: When Lorraine was recovered by France in 1919 it was decided not to return Briey to its former Department which is why, in terms of departmental boundaries, the town remains administratively separated from the eastern portion of the Briey Basin.
Split into simpler sentences: When Lorraine was recovered by France in 1919 it was decided not to return Briey to its former Department. Thus in terms of departmental boundaries, the town remains administratively separated from the eastern portion of the Briey Basin.