It is common to observe, based on congressional testimony and other public comments he has made, that Mattis has taken a hard line toward Iran, particularly the activities of the Revolutionary Guards and other allied or expeditionary Iranian militant units in Iraq, Syria, Lebanon, and Yemen. During our discussions, Mattis made a few comments along those lines. But mainly he seemed focussed on deepening America’s long-standing military and political alliances with Sunni Arab states—Morocco, Egypt, Jordan, Saudi Arabia, Qatar, and the United Arab Emirates. During his time at Central Command, he spent many hours talking to counterparts in those countries, which tend to view Shia revolutionary Iran as a serious threat. The smaller, militarily weaker Sunni states closest to Iran—such as the U.A.E.—were and remain acutely anxious that the United States might sell out their security in some Nixon-to-China grand bargain with Tehran.

I had the sense that, in Obama’s cabinet, Mattis channelled those Sunni Arab anxieties forcefully, but I never heard him itching for another Middle Eastern war or talking up the benefits of bombing Iran preëmptively. Over all, the Mattis in my notes seemed intently focussed on stability, wary of warfare that sought to promote democracy or idealism, sentimental about the independence of the Baltic states, firmly committed to nato, and unsentimental about Russia. During our stay in Estonia, he spoke publicly at a think-tank conference and made plain his commitment to the Baltic states’ membership in nato and the obligation to defend them from Russian aggression. He was particularly emotional about the role Estonian soldiers had played as nato members in Afghanistan and the sacrifices in lives they had made there.