To which Coenus, reputedly one of the most faithful Macedonians, replied, “If there is one thing above all others a successful man should know, it is when to stop.” Persuaded to turn around despite his fury at the mutineers, Alexander meandered with his army through India’s Gedrosian Desert and Iran for another three years before dying of fever in Babylon.

Afghanistan, as Major Portis reminded my class, presented insurmountable obstacles even to Alexander’s juggernaut. As he projected pictures of the rugged, stunning terrain of the Hindu Kush, he asked us to imagine what it was like for Alexander and the Mughal conqueror Babur, for the British, the Soviets and now the Americans to operate in an area that had never seen the end of war. “No one goes to this part of the world by accident,” he pointed out. “Yet a steady stream of foreigners has flowed through Afghanistan for centuries. Each group sought to impose their will on a people who now seem more resolute than the immutable mountains they call home. I get the feeling they won’t change for us either.”

The villages through which Major Portis and his unit moved were so isolated — “10 miles becomes an eternity with a 12,000-foot mountain standing in your way” — that his five interpreters, who spoke nine languages among them, could not understand the dialect. Finally, an interpreter and an elderly villager — one trained as an imam and the other in Shariah law — found a way to communicate imperfectly by exchanging memorized passages of the Quran.

Through this strange linguistic filter, Major Portis worked to understand the nuances of the region in which history had placed him. As he enumerated for us certain characteristics of Afghan campaigns — from spiraling casualties to troop surges to counterinsurgency — he stopped to acknowledge that we might be momentarily confused about which experience he was speaking of, Alexander’s or his own.

Major Portis commanded a cavalry troop at Combat Outpost Keating, which was almost overrun in a Taliban attack in October 2009. This battle, chronicled by Jake Tapper in “The Outpost,” was one of the war’s deadliest for Americans, with eight soldiers killed and 22 wounded. Away from the outpost when the attack began, Major Portis landed on the mountain above with a quick-reaction force, which fought its way down to Keating. But he didn’t visit us to talk about that battle or about beginnings or ends. Instead, he illuminated something of what it means to work, live and fight in the middle of things — in the middle of a war in the middle of a long and convoluted history of wars, deep within the mountains.

Major Portis is a practitioner, a man valued in military culture for his experience, but he came back to West Point to teach because he so forcefully believes in the necessity of studying with depth and care the stories of others to fill in the inevitable gaps in our experience. One of my students told him that the visit had given him some new perspective on the literature we were reading, but at the same time that literature was now helping the major to understand his war. Major Portis alluded to several things he wished he’d known before deploying.

Had he read accounts of Alexander’s march over the narrow passes of the Hindu Kush, he might have had an even richer appreciation for the challenges of operating in these mountains, especially in the winter. Had he read Babur’s 16th-century description of the region’s silver and lapis lazuli in “Baburnama,” he might have understood more readily the predicament of the Afghan miners who arrived at Keating attempting to sell stones for a song when the bottom fell out of the gem market. Had he read from the “Shahnameh,” he might have been more fully prepared for the diversity of religious rituals and cultural practices that characterizes this region.