Complex sentence: In early 2014, the group was reportedly composed of mostly non-Syrian Arab fighters, towards the end of that year, it had reportedly become a Syrian-majority group, partly because of defections from other rebel groups.
Split into simpler sentences: In early 2014, the group was reportedly composed of mostly non-Syrian Arab fighters. Towards the end of 2014, it had reportedly become a Syrian-majority group, partly because of defections from other Syrian rebel groups.