Carter is highly regarded and widely praised as effective in Washington
Fox will take over as acting deputy secretary of defence Thursday, Xinhua quoted Defence Secretary Chuck Hagel as saying. The administration is searching for a permanent replacement for Ashton Carter, who is to leave the job of deputy secretary of defence Wednesday.
The move was described by Marine Times, a military news outlet, as "unprecedented", highlighting the pressure the Pentagon is facing as it readies itself for a fight in Congress over defence budget in the coming months.
Fox, who until recently served as the Pentagon's director of cost assessment and programme evaluation, "helped identify the challenges, choices, and opportunities for reform facing the department during this period of unprecedented budget uncertainty, said a statement issued by Hagel.
Bringing back Fox, a seasoned budget expert, allows other senior defence officials to remain in their roles at a pivotal time as the Pentagon braces for a fight in Congress as next year's budget is set to be voted upon in the remainder of this year's session.
Carter is highly regarded and widely praised as effective in Washington. He played an important role in his job as the Pentagon 's number two, when Hagel, a career legislator, took over in February at the helm of one of the federal government's biggest agencies amid budget cuts and priority reassessment.
Fox left the Pentagon in June to become a senior adviser to the Johns Hopkins Applied Physics Laboratory, but remained an unpaid consultant to Carter, allowing her to keep current on defence issues.