Trump announced the early departure of Mattis in a tweet on December 23 and said that Shanahan would take over as of January 1.Shanahan worked for more than three decades at Boeing and was a senior vice president when he became Pentagon deputy in July 2017.
White House officials say Trump wants to focus on streamlining purchases at the Pentagon – an issue on which Shanahan has already been working.The officials said they didn’t know if Shanahan would be Trump’s nominee to replace Mattis.An acting defence secretary is highly unusual. Historically, when a secretary has resigned, he has stayed on until a successor is confirmed
