Complex sentence: The advance also coincided with the start of the winter rains, which not only affected the terrain, but also caused problems for the troops, who were only equipped for a desert war and lacked any winter clothing.
Split into simpler sentences: The advance also coincided with the start of the winter rains. Which not only effected the terrain, it also caused problems for the troops, who were only equipped for a desert war and lacked any winter clothing.