One witness the Senate ought to call when it sits to decide whether to confirm General Lloyd Austin as secretary of defense is General James Mattis. He’s the only living four-star general to have been given civilian leadership of the Pentagon. It turns out that General Mattis, in his book “Call Sign Chaos,” has something to say about General Austin — and about Mr. Biden — that gives pause.

It raises, at least in our mind, questions about whether General Austin is the type who would give the civilian leadership unvarnished advice. And, more broadly, whether, as a civilian leader, he would be too political and unwilling to credit the advice of the uniformed officers under his authority. Plus, too, it could provide insights into the modus operandi of Joseph Biden, who is nominating him.

We’re certainly no insider on this beat. Just the other day, though, Mr. Biden issued in the Atlantic an explanation of why he tapped General Austin. Eli Lake of Bloomberg has a terrific column on it. Mr. Biden begins by describing how in August 2010 he flew to Baghdad for the change-of-command ceremony at which General Austin assumed command of all our forces in Iraq at a moment of retreat. General Mattis and Mr. Biden had a different view of things.

General Mattis’ account in “Call Sign Chaos” is in a chapter called “Snatching Defeat From the Jaws of Victory.” General Mattis had flown to Baghdad for the same change of command ceremony Mr. Biden attended. General Mattis writes of how the outgoing commanding general in Iraq, Ray Ordierno, had given Secretary of Defense Gates and General Mattis a “carefully calculated plan” to leave a residual force.

That plan was for 18,000 troops. After the change of command ceremony, our ambassador, James Jeffrey, invited Vice President Biden and the Yank generals to dinner. Among the things that General Mattis told Mr. Biden was his concerns about Iraq’s efforts to “purge or marginalize” the Sunnis and Kurds. “Prime Minister Maliki is highly untrustworthy, Mr. Vice President,” General Mattis told Mr. Biden.

Yet as he spoke, General Mattis writes, he gained the impression that, though the vice president was listening politely, it was “like talking to people who lived in wooden houses but saw no need for a fire department. I saw that the die was already cast.” Mr. Biden told him: “Maliki wants us to stick around, because he does not see a future in Iraq otherwise.” Added Mr. Biden: “I’ll bet you my vice presidency.”

After dinner, as General Mattis tells the story, the vice president was in a joshing mood. “Know why you’re at CENTCOM?” Mr. Biden teased General Mattis. “Because no one else was dumb enough to take the job.” Adds General Mattis: “I found him an admirable and amiable man. But he was past the point where he was willing to entertain a ‘good idea.’ He didn’t want to hear more; he wanted our forces out of Iraq.”

“Whatever path led there fastest, he favored,” writes General Mattis of Mr. Biden. “He exuded the confidence of a man whose mind was made up, perhaps even indifferent to considering the consequences were he judging the situation incorrectly.” We take this to be what Governor Sarah Palin foresaw when, in the 2008 vice presidential debate, she accused Mr. Biden of “waving the white flag of surrender.”

Which brings us back to General Austin. He agreed, in General Mattis’ telling, with General Ordiano’s recommendation of 18,000 troops remaining in Iraq. Yet General Mattis includes a footnote to one account that says that General Austin “made clear that he was more comfortable with the status quo,” whereas General Mattis “was not.” And we just wonder what the episode tells us.

Does General Mattis reckon General Austin is the right man for the job? How does General Mattis’ fury with President Trump for abandoning the Kurds in northern Syria compare with his obvious distaste for the way Mr. Biden precipitated the retreat from Iraq? And what does General Matthis reckon his own career in a uniform tells us about how a recently-uniformed four star might do in a civilian role?