"We are substantially subsidising the militaries of many very rich countries all over the world, while at the same time these countries take total advantage of the United States, and our taxpayers, on trade, " he Tweeted.
Shanahan, 56, is a native of Washington State who holds an undergraduate degree from the University of Washington and two graduate degrees from the Massachusetts Institute of Technology.
A number of names have been floated in recent months, as rumors of Mattis' impending departure have been circulating since well before Trump referred to him as "sort of a democrat" in a recent interview.
It is unusual for the Pentagon to have an acting secretary of defense.
Shanahan, in his job as deputy defense secretary, has largely focused on internal Pentagon reform and issues like the creation of a Space Force, a project championed by Trump but resisted by some lawmakers and some in the Pentagon. Historically when a secretary has resigned, he has stayed on until a successor is confirmed.
Outgoing US Defence Secretary James Mattis has signed an order withdrawing US forces from Syria, a Defence Department spokesman said on Monday, following through on a decision by President Donald Trump that helped trigger Mattis' resignation last week.
Kate Middleton spotted at The Range in King's Lynn
According to Daily Mail , Kate affectionately called Charlotte "Poppet" when asking her get up off of the floor of the store. She [The Queen] was very concerned that it [the Markle situation] was spiraling out of control, which it was.
The official said Trump was expected to pick a nominee for defense secretary over the next couple of weeks.
"I've encouraged him to find people who have some overlap with him but don't see the world in lockstep with him", Mulvaney said.
President Trump expounded on his differences with Secretary Mattis on the day before Christmas.
Trump made the decision to withdraw US forces from Syria and pull out about half of the 14,000 USA troops in Afghanistan, against the advice of his top aides and USA commanders.
Trump made the decision last week to withdraw United States forces from Syria and pull out about half of the 14,000 USA troops in Afghanistan, against the advice of his top aides and U.S. commanders.
They have asked Trump to wait 90 days before making a decision, but Mike Mulvaney told ABC there isn't a chance Trump will change his mind.
On Dec. 14, as he's done about a dozen times since taking office, Trump phoned Turkey's strongman president, Recep Erdogan.
The US has for years supported the Kurdish-led Syrian Democratic Forces (SDF) in the fight against IS in Syria. Let's hope that we see more of that in the New Year.