Complex sentence: The sudden increase in bomber losses had often been attributed to flak but initial reports from air gunners of German night fighters stalking their prey from below had appeared as early as 1943, but had been discounted.
Split into simpler sentences: The sudden increase in bomber losses had often been attributed to flak. Initial reports from air gunners of German night fighters stalking their prey from below had appeared as early as 1943, but had been discounted.