Complex sentence: They were delayed by 20 minutes and the smoke screens that should have hidden their assault had lifted, with the advantage of surprise and darkness lost the Germans had manned their defensive positions in preparation for the landings.
Split into simpler sentences: They were delayed by 20 minutes and the smoke screens that should have hidden their assault had lifted, eliminating the advantage of surprise and darkness lost. The Germans manned their defensive positions in preparation for the landings.