Trump said, lamenting the ongoing Afghan security crisis and the amount America spends in the war-torn nation. "But, as you know, President (Barack) Obama fired him and essentially so did I", Trump said.
In his Cabinet meeting remarks with reporters present, Trump said he was in no rush to get USA troops out of Syria but did not clarify a timetable.
Mattis stepped down as defense secretary last month over differences with Trump on a host of national security issues, particularly Trump's decision to withdraw US troops from Syria.
Mattis, who disagreed with the plan, offered his resignation after failing to convince Trump to reverse course on the pull-out.
The drama came after Shanahan sought to lay out his priorities for America's massive defense department and how he intends to deal with an urgent list of geopolitical to-dos, including the U.S. withdrawal from Syria.
"We all hear about how our competitiveness has eroded", Shanahan said. The US official, who spoke on condition of anonymity, did not elaborate on Shanahan's views on China or what other guidance he gave during the meeting.
Cuomo reportedly asked Elon Musk's Tesla to help fix NYC's subway system
He said not fully closing the L train would be a "phenomenal benefit to the people of New York City". Since 1990, ridership has vastly increased, with about 400,000 rides on an average day.
Seth Jones at the Center for Strategic and International Studies think-tank in Washington said Shanahan started the job from a somewhat weaker position, since he did not have the mandate that comes with being a Senate-confirmed defense secretary. The two have worked closely on the fiscal 2020 budget submission, which Shanahan has repeatedly pledged will be a "masterpiece" that puts the National Defense Strategy into motion.
Acting Defense Secretary Patrick Shanahan told the military service chiefs Wednesday that dealing with China would be his top priority.
The strategy singled out China and Russian Federation as the main threats to US national security, as opposed to the counter-terrorism focus of past formulas for the overall direction of the Defense Department.
Shanahan, who had served as deputy to Mattis since July 2017, is a career business executive with no prior government or military experience.
The Trump administration has had a troubled relationship with China.
David Norquist, the Pentagon's comptroller, will perform the duties of the deputy as well as retain his current title.
"The National Defence Strategy is [Mattis'] strategy", Shanahan said.