WASHINGTON — In November 2012, shortly before the White House ended his tenure commanding U.S. forces in the Middle East, Marine Gen. James Mattis delivered an urgent message to his boss: An Iranian fighter jet had fired errantly on a U.S. drone overflying the Persian Gulf.

In considering how the U.S. should respond, Defense Secretary Leon Panetta took into account a central feature of Mattis’s reputation: his nerve.

“I could sense that Mattis did not want to back down,” Panetta recounted in his memoir, “Worthy Fights.” “And that the White House was wary of his resolve. As I already knew, the White House didn’t fully trust Mattis, regarding him as too eager for a military confrontation with Iran.”

The crisis faded. But Mattis now stands on the brink of becoming Pentagon chief for a president-elect who has pledged to toughen U.S. policy toward Iran. And that could have broad implications as the incoming administration weighs trying to modify the 2015 Iran nuclear deal and American posture in the Middle East after a litany of complaints from U.S. allies that President Barack Obama yielded too much ground to Tehran.

Having retired in 2013, Mattis would be the first career military officer to serve as defense secretary in more than a half-century. He will testify Thursday before the Senate Armed Services Committee, which is expected to broadly approve his nomination.