Mattis offered to stay on the job until the end of February to give Trump time to consider a replacement, but the president ordered him out by January 1.
Shanahan has never served in the military, but he was a top executive at Boeing Co.
"I am particularly distressed that he is resigning due to sharp differences with the president", the Kentucky Republican said in a statement.
The tweet was a markedly firm defense of the president's announcement last week that he would pull all US troops out, shocking top USA officials, allies, and partners. Since then, no announcements have been made, and senior US military officers have said they have received no new orders. There is also concern about abandoning the U.S.' Kurdish allies. On Sunday, Trump announced via Twitter that Mattis would leave office two months earlier than anticipated and be replaced by Patrick Shanahan, the current deputy secretary of defense, upon his departure.
White House considers ban on Chinese 5G technology
Australia and New Zealand have banned Huawei from building their 5G networks on national security grounds in the last few months. In fact, Huawei's vice president for sales of Huawei Tech USA, is on the board of directors of the Rural Wireless Association.
Actor Kevin Spacey's encounter with teenager detailed in court records
While on his way home, Spacey texted him, "I think we lost each other", and the busboy did not respond, the complaint states. Kevin Spacey speaking at the Bits and Pretzels founders' and investors' festival in Munich, Germany in September 2017.
Russian weapon 27 times faster than speed of sound
The intercontinental, nuclear-capable hypersonic missile system will enter service in Russia's military next year, he added . The new weapon "essentially makes missile defences useless", he said.
Trump pressed White House national security adviser John Bolton to make the move December 18, and he and other US officials were trying to talk him out of it, administration officials said. Fox News has confirmed that McGurk submitted his resignation letter on Friday, a day after Mattis resigned.
Tom Mattis, who lives in their hometown of Richland in Washington state, said James Mattis was not angry about the fiasco, which was prompted by Trump's decision to completely withdraw all US forces from Syria.
The spokesperson for the Russian Foreign Ministry said on Wednesday the planned withdrawal of half of American troops from Afghanistan was "a step in the right direction".
The US troops in Afghanistan are divided between training and advising Afghan forces and a counterterror mission. More than 2,400 American soldiers have died in Afghanistan since the 2001 USA -led invasion, and Trump announced in a video last week that troop deaths were on his mind in announcing the pullouts.