Secretary of State Rex Tillerson has reassigned a majority of the staff meant to work most closely with the top US diplomat in what career officials at the State Department fear is the start of a major reorganization. The news sent shock waves through the agency and has left career officials on edge, in part because of its abrupt nature — taking place before their assignments end this summer and replacements have been found — and in part because these officials help the secretary, a government novice, work with policy experts throughout the building.