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Before the start of business, Just Security provides a curated summary of up-to-the-minute developments at home and abroad. Here’s today’s news.

JIM MATTIS RESIGNATION

U.S. Defense Secretary James Mattis will leave his post before 2019, with Deputy Secretary of Defense Patrick Shanahan leading the Pentagon as acting secretary effective Jan. 1. President Trump announced yesterday. “I am pleased to announce that our very talented Deputy Secretary of Defense, Patrick Shanahan, will assume the title of Acting Secretary of Defense starting January 1, 2019,” the president wrote in a message sent on Twitter, adding “Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing … he will be great!” Quint Forgey reports at POLITICO.

Mattis’ new departure date comes two months earlier than expected and follows his resignation Thursday, announced in a parting letter pointing to differences between himself and the president in the wake of Trump’s decision to pull all U.S. troops out of Syria. Michael R. Gordon, Nancy A. Youssef and Jessica Donati report at the Wall Street Journal.

On receipt of the letter – but before reading its contents – the president had dictated a “glowing” tweet announcing that Mattis was retiring “with distinction” at the end of February. The president has reportedly grown increasingly angry as in subsequent days as he watched a series of defense analysts take to television to extol Mattis’ courage, Helene Cooper and Katie Rogers report at the New York Times.

It is possible that the president and other advisers – eager to retaliate following Mattis’ critical parting letter – also suspected Mattis of being part of a campaign to generate anti-Trump press coverage, according to senior administration officials. The move brings fresh instability to the Pentagon as it navigates Trump’s abrupt decisions to withdraw U.S. troops from Syria and Afghanistan. Philip Rucker, Dan Lamothe and Josh Dawsey report at the Washington Post.

Shanahan was confirmed by Congress as Deputy Secretary of Defense in July 2017 and has essentially served as chief operating officer for the Pentagon. The former Boeing executive has previously told NPR’s Steve Inskeep that new technology will shape the future of war, commenting: “the investment in weapons systems is long term … so when we watch, whether it’s the Chinese or the Russians or what extreme terrorist organizations are doing, we have to put an eye towards what are those capabilities where we want to extend our advantage,” Francesca Paris reports at NPR.

JIM MATTIS RESIGNATION: OPINION AND ANALYSIS

“The question was always when Mattis would reach his breaking point,” Blake Hounshell comments at POLITICO Magazine, arguing that President Trump’s refusal to be counseled made Mattis’ departure inevitable.

Mattis was a “scholar of war,” former Army officer John A. Nagl writes at the New York Times, reminiscing about Mattis’ record in Iraq and concluding that the U.S. “will be less safe, and our enemies greatly heartened, by his departure.”

Mattis’ resignation “signals the end of a powerful myth in Washington—that a team of national security advisers could form a so-called ‘axis of adults,’” and the president’s remaining team of advisers will enable him to pursue a truly novel foreign policy agenda, Ivo Daddler and James Lindsay comment at POLITICO Magazine.

Mattis may have been ‘the adult in the room’ but he never offered a material on “an errant president,” Jonathan Stevenson comments at the New York Times.

Despite all of Mattis’ strengths “he encouraged some habits in the Pentagon that don’t augur well for future administrations or civil-military relations,” Dereck Chollet explains at Foreign Policy.

In Patrick Shanahan – Trump may have found a like-minded proponent of the U.S. weapons industry, David Axe explains in an analysis of the former Boeing executive’s background and policy positions at The Daily Beast.

Five possible successors to Mattis are outlined by Ellen Mitchell and Rebecca Kheel at the Hill.

MIDDLE EAST TROOP WITHDRAWAL

The U.S. military has confirmed that the order to withdraw American troops from Syria had been signed, following President Trump’s decision last week. “The execute order for Syria has been signed,” a U.S. military spokesperson told AFP yesterday without providing further details, AFP reports.

Trump claimed yesterday that he had spoken on the phone with his Turkish counterpart Recep Tayyip Erdoğan regarding the withdrawal of U.S. troops from Syria. “We discussed I.S.I.S [Islamic State Group] our mutual involvement in Syria, and the slow and highly coordinated pullout of U.S. troops from the area … after many years they are coming home,” Trump said in a message sent on Twitter, also claiming that he and Erdoğan also discussed “heavily expanded” trade between the U.S. and Turkey following the deterioration in Washington-Ankara relations over the summer, Al Jazeera reports.

“President @RT_Erdogan of Turkey has very strongly informed me that he will eradicate whatever is left of I.S.I.S. in Syria….and he is a man who can do it plus, Turkey is right ‘next door,’” Trump wrote in a later message, adding: “our troops are coming home!” Haaretz reports.

U.S. special presidential envoy for the Global Coalition to Defeat Islamic State Group Brett McGurk has quit over the decision to pull troops from Syria, bringing his departure forward from February. Trump took to Twitter to underplay the development, sending a message stating: Brett McGurk, who I do not know, was appointed by President Obama in 2015 …was supposed to leave in February but he just resigned prior to leaving … Grandstander? The Fake News is making such a big deal about this nothing event!” the BBC reports.

French President Emmanuel Macron yesterday criticized Trump over the Syria decision, saying “an ally should be dependable,” and adding that fighting “shoulder to shoulder” is the “most important thing for a head of state.” Macron told a news conference that “I very deeply regret the decision made on Syria,” also emphasizing the work of the U.S.-backed Kurdish forces – “I call on everyone … not to forget what we owe them,” Michael Burke reports at the Hill.

“It was a good week for [Islamic State Group] …. the pressure is off in Syria … while its vicious urban bombing campaign in Afghanistan prospectively faces less resistance,” Simon Tisdal writes at the Guardian in an analysis of the impact of Trump’s decisions last week to pull U.S. troops from Syria and Afghanistan.

“Withdrawing half our forces flies in the face of pursuing a conditions-based approach and would reduce the credibility of any new strategy announcements,” Samantha Vinograd comments on Trump’s Afghanistan decision in her “weekly briefing” to the president at CNN.

Although several commentators were quick to compare Trump’s action with President Obama’s decision to withdraw U.S. forces from Iraq in 2011 – “the two cases are far from identical,” Brian P. McKeon explains at Just Security, noting that “understanding their differences helps to show what makes President Trump’s action so dangerous.”

Trump’s political isolation over the Syrian and Afghan decision is in fact the president’s desired posture, Jonathan Allen writes at NBC, arguing that “Trump doesn’t want to be aligned with the foreign policy consensus in Washington or around the world.”

SYRIA: OTHER DEVELOPMENTS

Turkey is reportedly massing troops near the northern Syrian town of Manbij held by the U.S.-backed Kurdish-led Syrian Democratic Forces (S.D.F.), U.K.-based monitor the Syrian Observatory for Human Rights said as Turkish media reported yesterday new reinforcements crossing the borders. The Turkish military consolidation comes even though Turkey said it would postpone a promised offensive in eastern Syria following Trump’s decision this week to withdraw U.S. troops from the war-torn country, Sarah El Deeb and Zeynep Bilginsoy report at the AP.

U.S.-led airstrikes continue. U.S. and coalition forces carried out 208 airstrikes against Islamic State targets in Syria between Dec 9. and Dec. 15. [Central Command]

TRUMP-RUSSIA

President Trump is jeopardizing “the rule of law” by discussing his legal situation with acting Attorney General Matthew Whitaker, incoming chairman of the House Intelligence Committee Rep. Adam Schiff (D-Calif.) said yesterday. Speaking on CNN’s “State of the Union,” Schiff claimed that reports that Trump has expressed frustration to Whitaker about federal prosecutors in New York indicated wrongdoing at the very top of the Trump administration, adding: “the president of the United States is discussing a case in which he is implicated with the attorney general … that is wrong at every level … and, of course, it will taint anything that this acting attorney general does, any role he plays in this investigation …this is a real assault on the rule of law,” David Cohen reports at POLITICO.

A close analysis of the Department of Justice’s (D.O.J.) letter explaining why Whitaker will not recuse himself from overseeing Mueller’s investigation, highlighting the glaring omissions in the D.O.J.’s reasoning, is provided by Editor Marty Lederman at Just Security.

Special counsel Robert Mueller – investigating Russian electoral interference and alleged collusion by the Trump presidential campaign – has enjoyed a year of successes, Tom McCarthy comments at the Guardian, outlining various measures of the probe’s success and suggesting that “2019 could be even stronger” for Mueller’s prosecutors.

U.S.-RUSSIA RELATIONS

Russian operations intended to polarize U.S. voters continued during the 2018 midterm elections but did not compromise voting systems used, according to a study by the intelligence community. The assessment by Director of National Intelligence Dan Coats comes as a result of a prior request by the White House that Coats examine election meddling by Russia and other powers; the agency has not released the report, but Coats released a statement on the document, claiming that “Russia, and other foreign countries, including China and Iran, conducted influence activities and messaging campaigns targeted at the U.S. to promote their strategic interests,” Julian E. Barnes reports at the New York Times.

News of turmoil amid U.S. national security circles following last week’s developments has provoked delight in Russia. Neil MacFarquhar explains at the New York Times.

OTHER DEVELOPMENTS

A U.N. advance team arrived in the Yemeni port city of Hodeidah yesterday to commence monitoring a ceasefire and withdrawal of forces agreed by the Iran-aligned Houthi rebel group and Saudi-backed government forces, according to U.N. sources. Reuters reports.

Tesla founder Elon Musk’s SpaceX launched a military satellite yesterday from Florida’s Cape Canaveral as the space exploration company began its first national security mission. The G.P.S. III satellite built by Lockheed Martin reportedly launched at 8:51 a.m. Eastern; SpaceX 2016 won an $83 million Air Force contract in 2016 to launch it, Michael Burke reports at the Hill.

An Op-Ed on “the unlearned lessons of [Washington Post columnist] Jamal Khashoggi’s murder,” is provided by the Washington Post editorial board.