Complex sentence: Before the units could get to their rest areas, their trains and trucks were stopped and they were called back to the front to participate in an attack that was expected to relieve some of the pressure on Teruel.
Split into simpler sentences: Before the units could get to their rest areas, their trains and trucks were stopped and they were called back to the front. They participated in an offensive that was expected to relieve some of the pressure on Teruel.