Complex sentence: Adm. William Halsey, Jr. was lured into taking his powerful 3rd Fleet after a decoy fleet, leaving only three escort carrier groups of the 7th Fleet in the area.
Split into simpler sentences: Adm. William Halsey, Jr. was lured into taking his powerful 3rd Fleet after a decoy fleet. Thus only three escort carrier groups of the 7th Fleet remained in the area.