ARLINGTON, VA - NOVEMBER 09: U.S. Secretary of Defense Jim Mattis (L) hosts an honor cordon ceremony to welcome China State Councilor and Defense Minister General Wei Fenghe at the Pentagon, on November 9, 2018 in Arlington, Virginia. The two planned to discuss "risk reduction" in an effort to limit the possibility of inadvertent clashes. (Photo by Mark Wilson/Getty Images)
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Politics of the Day 16 videos
ARLINGTON, VA - NOVEMBER 09: U.S. Secretary of Defense Jim Mattis (L) hosts an honor cordon ceremony to welcome China State Councilor and Defense Minister General Wei Fenghe at the Pentagon, on November 9, 2018 in Arlington, Virginia. The two planned to discuss "risk reduction" in an effort to limit the possibility of inadvertent clashes. (Photo by Mark Wilson/Getty Images)
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WASHINGTON, DC - DECEMBER 01: Rep. George Santos (R-NY) is surrounded by journalists as he leaves the U.S. Capitol after his fellow members of Congress voted to expel him from the House of Representatives on December 01, 2023 in Washington, DC. Charged by the U.S. Department of Justice with 23 felonies in New York including fraud and campaign finance violations, Santos, 35, was expelled from the House of Representatives by a vote of 311-114. Santos is only the sixth person in U.S. history to be expelled from the House of Representatives. (Photo by Kevin Dietsch/Getty Images)
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WASHINGTON, DC - DECEMBER 01: Rep. George Santos (R-NY) is surrounded by journalists as he leaves the U.S. Capitol after his fellow members of Congress voted to expel him from the House of Representatives on December 01, 2023 in Washington, DC. Charged by the U.S. Department of Justice with 23 felonies in New York including fraud and campaign finance violations, Santos, 35, was expelled from the House of Representatives by a vote of 311-114. Santos is only the sixth person in U.S. history to be expelled from the House of Representatives. (Photo by Kevin Dietsch/Getty Images)
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Editor’s Note: Julian Zelizer is a professor of history and public affairs at Princeton University, and author, with Kevin Kruse, of the forthcoming “Fault Lines: A History of the United States Since 1974.” The opinions expressed in this commentary are his own. View more opinion at CNN.

CNN  — 

Secretary of Defense James Mattis is stepping down at an extraordinarily fraught moment in American foreign policy. Coming in the wake of President Trump’s surprise decision to withdraw troops from Syria, without having consulted allies and after barely talking to his own national security advisers, Secretary Mattis becomes the latest “adult in the room” to leave.