He's referring to Halsey's actions at the Battle of Leyte Gulf - Halsey was supposed to be covering the landing force at Leyte but instead was drawn off by a Japanese decoy force, leaving the landing largely unprotected. He compounded his decision with a series of unclear communications that left other American forces assuming that he had left his battleships to cover the landings while he took the rest of his ships north (in fact he had taken them with his carriers).

Per /u/kieslowskifan on an older topic the reference to Leyte is more explicit in the book.