The possibility that retired general James Mattis could become Donald Trump’s defense secretary is virtually the only good news to emerge out of the chaos of the presidential transition.

Mattis commanded a Marine battalion during the Persian Gulf War and a Marine regiment during the Iraq invasion, and he later headed the U.S.’s Joint Forces Command and U.S. Central Command during some of the most pivotal years of the U.S. campaigns in Iraq and Afghanistan. Hawks admire Mattis because of his , but he has fans on the other side of the spectrum as well, such as realism-minded centrist types and .

Even people further left can find something they like in Mattis’s record— saying he “paid a military security price every day as the commander of CentCom because the Americans were seen as biased in support of Israel.”

A true careerist might avoid connecting their fortunes to an already unpopular president-elect who got 2.5 million fewer votes than his opponent, but Mattis might see a patriotic responsibility to serve anyway. If Trump can’t close the deal with him, or just can’t stand the thought of someone with infinitely greater gravitas than him serving in his administration, it could be the latest sign of the president-elect’s myopia, or of a larger problem of getting credible people to work for him.

The next defense secretary will be confronted with a related challenge. Donald Trump has grotesquely boasted of being “” and said that generals would have to listen to him , like killing the families of terrorists, something which Trump . The military officers and civilian officials at the Pentagon , and many of them have lucrative career prospects outside of public service. The incoming SecDef must convince the department’s mid and even top-level leadership that the next administration won’t be an erratic pseudo-criminal enterprise, thus stanching any potential exodus to the private sector.

At least one expert has raised doubts over whether Mattis is even all that well suited to managing the Pentagon, never mind at a sensitive moment like this one. Erin Simpson, who worked with Mattis while advising the U.S. mission in Afghanistan, wrote in that the general “is not a politician, or a wonk, or a bureaucrat. To ask him to be any of those things would be like trying to keep a wave upon the sand.”

Simpson also notes that Mattis would need a congressional waiver to serve as SecDef, since the law prohibits military personnel from filling the position until seven years after retirement. But Mattis would at least come into the office with reservoirs of credibility and goodwill, especially in light of the current, perfectly reasonable low-level national panic over what a Trump presidency actually means for the country.

Mattis is a legendary figure among Marines and perhaps the most revered Marine of the entire post-Cold War era. Mattis never remained aloof from the rank-and-file and would deployed to risky combat positions in Afghanistan. He’s one of the few active American public figures to have uttered entire - of . He sends viral emails about how , and I can personally confirm that it is.

He’s viewed as the epitome of what a U.S. Marine can and should be, something which a second career in the cattle-trough of Trump-era American politics would inevitably complicate or taint. A tour of the E-Ring of the Pentagon—on behalf of Donald Trump, let’s remember—is likely to shrink Mattis right back down to human proportions.

And then there’s the most consequential and controversial aspect of Mattis’s legacy, something that the Obama-era shift in the U.S. foreign policy orientation and the domestic, Trump-driven reordering of American politics have memory-holed by now. Mattis was one of the architects of American counterinsurgency strategy, as well as one of the chief theoreticians of US counterinsurgency (COIN) doctrine.

Along with General David Petraeus——Mattis spearheaded the 2006 update to the . The re-think was partly in response to the U.S. flailing campaign in Iraq, although the military’s thinking on the topic was already badly out of date, with the last previous update to the manual dating to the mid-1980s.

The new COIN doctrine posited that the key to the achievement of US objectives in an asymmetrical conflict against a non-state force was securing the civilian population that the enemy most actively threatened, but that (somewhat paradoxically) also provided that enemy with its base of support. In Iraq, this largely meant encouraging the emergence of anti-al Qaeda Sunni militant movement called the Sons of Iraq, or Sahwa—which sometimes meant to former extremists who had themselves fought against the U.S. military at some earlier point in the war.

It also meant greatly expanding the U.S. military’s concept of its role in the country, since improving and thus securing civilian life . COIN often demanded that the army operate like a militarized development agency, as well as a fighting force.

COIN also entailed some unsavory political tradeoffs whose costs were inherently unknowable, both at home and abroad: COIN is manpower-heavy and required controversial “surges” of US forces to help the strategy gain traction in either Iraq and Afghanistan. The Sahwa helped neutralize al-Qaeda in Iraq as long as the US Military was in the country to support them. But after the 2011 American pullout from the country, the pro-Shi’ite government of Iraqi premier Nouri al-Maliki that the Sahwa could be the basis for a future sectarian conquest of Baghdad and undermined the Sunni militias that had fought Sunni jihadists during the US occupation.

When ISIS emerged two years years later, a second Sahwa did not materialize to oppose them, and Iraq is now as riven with sectarian militancy as it’s ever been.

By the end of the 2000s, COIN was poised to usher in a new era in American military thinking. But these days, now read like a relic of a different time. COIN played a part in temporarily pacifying Iraq, but it had , where Petreus commanded US and allied forces in 2010 and 2011.

The U.S.’s 2011 military campaign in Libya involved no COIN and no American ground troops. When ISIS took over swaths of Iraq and Syria, necessitating the deployment of several thousand US troops to Iraq, COIN was little in evidence, as the US-led coalition focused on high-value targets and ISIS infrastructure while leaving most of the ground combat to Iraq’s national military and Shi’ite militias, forces with notably little concern for the security of civilian populations.

Maybe COIN was never as revolutionary as it seemed to many of its supporters– after the ordeals of Iraq and Afghanistan, the American public and its leaders were bound to turn against large-footprint ground operations and to think of the country’s military responsibilities in more limited terms. As a specific response to a specific problem, COIN arguably worked, as it allowed the U.S. to withdraw from an Iraq that appeared to be relatively peaceful and secure by the early 2010s. And it’s unfair to dump the very complex history of COIN on to one person, however prominent.

Still, one hopes that members of the Senate Armed Services Committee spend some time picking Mattis’s brain on COIN, not to trap or flummox him, but to get an idea of where his own thinking about warfare has changed over the past decade. Is COIN still applicable to the US’s various security challenges? Could Islamist or even jihadist anti-regime elements in Syria be co-opted into fulfilling U.S. objectives the same way Sahwa militants were in Iraq—and would such a thing even be desirable in the long run? How prepared should the U.S. military be to carry out another COIN-centric, troop and treasure-heavy Iraq War-like mission?

Mattis’s confirmation shouldn’t necessarily depend on how he answers these questions, but the hearings provide an unusually public opportunity to probe these still hotly contested issues. (, however, are likely to arise during those hearings.)

There’s another, perhaps more impudent question the senators should consider asking, something along the lines of: How can we expect someone like you to successfully work for someone like Trump? In 2010, about a platoon in Iraq’s Anbar province that had “showed kindness to the civilians caught in the crossfire” even after suffering heavy losses. “They had just finished scraping up their buddies off the deck but showed the people respect,” Mattis recalled. Dickerson, who interviewed the Marine general at his home in Norfolk, Virginia, wrote that Mattis’s bookshelves “are packed with histories and military manuals,” and added that during their conversation the general would “regularly get up to retrieve a volume—to cite a passage about the insurgency in Algiers or show a table about fuel use in the initial sprint into Iraq.”

Mattis has spent a lifetime in the war business without becoming desensitized to its human costs, and without growing so hardened, cynical, or arrogant as to lose his intellectual curiosity. His would-be boss is of a very different breed. How do you think that’s going to work out?

is a New York-based writer and editor who has reported from throughout Africa and the Middle East.