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.