Complex sentence: The first phase of the offensive ended successfully, after the capture of 15 towns, villages and hills, but it slowed soon after and stalled during attempts to advance on Kafr Shams and Kafr Nasij.
Split into simpler sentences: The first phase of the offensive ended successfully, after the capture of 15 towns, villages and hills. But the operation slowed soon after and stalled during attempts to advance on Kafr Shams and Kafr Nasij.