Complex sentence: New trenches collapsed as they were dug and the front and support lines were held by shell-hole posts, which became islands of squalor, as duckboards and ration boxes used as platforms sank under the mud; cooking became impossible and movement in daylight suicidal.
Split into simpler sentences: New trenches collapsed as they were dug and the front and support lines were held by shell-hole posts, which became islands of squalor. Duckboards and ration boxes used as platforms sank under the mud; cooking became impossible and movement in daylight suicidal.