Complex sentence: He became known by the moniker "Legend" amongst the general infantry and Marines with whom he was tasked with protecting due to his unusually high kill total established during the early weeks of his first tour.
Split into simpler sentences: He became known by the moniker "Legend" amongst the general infantry and Marines with whom he was tasked with protecting. This title being the result of his unusually high kill totals established during the early weeks of his first tour.