On August 21, 2018, General John Nicholson passed command of NATO’s Afghanistan mission to Lieutenant General Scott Miller. In his final press conference, Nicholson stated the strategy is working and just needs more time. He was the seventeenth commander of Afghanistan and his parting statement sounds very much like the previous sixteen. Given some of the most successful military leaders of this generation have command there, why does Afghanistan still remain unstable?

Clearly, Afghanistan has a very along history of ejecting foreign forces. On January 13, 1842, Assistant Surgeon William Brydon, bloodstained and exhausted, reached the British Fort at Jalalabad. When asked where the rest of the army was, he managed to reply “I am the Army.” Thus the British learned their 20,000-man army in Afghanistan had been wiped out. Though it is perhaps the most famous example of a Western army being defeated by a mountain people, it is certainly not the only one.

From Alexander’s three year campaign (329 to 327 BC) to control Bactria (what is now Afghanistan) to the present, military history is littered with of stories of major powers, confident of victory, venturing into the territory of mountain people—only to be ejected.

The English first began serious efforts to subdue Scotland in the twelfth century, but it took centuries of fighting before the Act of Union joined the two countries in 1707. Even this did not end Scottish resistance as the Scots rebelled in 1715 and 1745—and argue about independence to this day. The Russians have been fighting on and off in the Caucasus since the early 1700s, and still struggle to suppress terrorist groups in the region. The Maronite Christians of Lebanon have held their mountains against Muslims for over a thousand years.

Outside powers can win against mountain people but it takes decades to centuries. Afghans, Chechens, Kurds, Montagnards (which literally means “mountain people” in French), Scots, Welsh, Swiss, Druze and Maronite Christians have all repeatedly seen off outsiders. Although the Scots and Welsh were finally integrated into the United Kingdom, it took centuries to conquer each nation. Most importantly, the English had strong strategic reasons for pursuing these campaigns. First, the English kings truly believed the territories rightfully belonged to them. This drove the over two century-long effort of the English to conquer Wales. Later, as England faced continental enemies, it could not accept an independent Scotland that repeatedly allied with continental foes. Thus, England was willing to pay a high price to finally subjugate the Scots.

Virtually every mountain society has stories of outside invaders turned away. These stories form a central element in the people’s identities. Also central to mountain identities are the long-running internal feuds. Families and clans have engaged in disputes that have lasted for centuries. Inevitably outsiders who enter the mountains get drawn into these feuds although they rarely understand them. In her book No Friends but the Mountains, Judith Matloff takes the reader on an intimate tour of mountain societies from the Sierra Madre to the Caucasus to the Himalayas and Andes. She notes that while mountains contain only 10 percent of the world’s population, they were home to twenty-three of the twenty-seven wars at the time of her writing. She also highlights the blood feuds that complicate governance in the mountains.

Switzerland, now seen as one of the most stable, democratic and prosperous nations in the world, took centuries to work out its internal government issues. First formed in 1291 by an alliance of three cantons, it was not until 1848 the Swiss agreed to unify under a single government. Prior to that, there was a great deal of internal conflict. Even today, the twenty-six cantons and three thousand communes (municipalities) retain a great deal of independence in deciding local issues. Even as a modern, highly urban society, the presence of three thousand communes in a tiny nation indicates the Swiss retain the mountain culture’s desire for local governance. It is also important to realize that outsiders will often not like their decisions because they will not reflect outside values. For instance, Swiss women did not get the right to vote in federal elections until 1971. The last canton to grant women the right to vote on local issues, Appenzell Ausserrhoden, only did so in 1991 and only when ordered to by the Federal Supreme Court of Switzerland.

Clearly, “Leave mountain people alone” should be a rule of thumb at least as prominent as “Never fight a land war in Asia.” But it isn’t. It is time war colleges integrate this rule of thumb into their curriculums. Politicians commit the nation to war, not military officers. But it is the military officers that assure the politicians they can win. Despite seventeen years of campaigning by American, allied and Afghan national forces, security in Afghanistan continues to deteriorate. Perhaps if U.S. decisionmakers understood they were was entering a fight with mountain people, their expectations would have been more realistic.

Yet, if the rule is to be applied, an obvious question is how to define mountain people. Just because people live in a mountainous area does not mean they are mountain people. I define mountain people as those who make their living in the mountains rather than in the valleys. For instance, Korea and Japan are mountainous, but are also rice cultivating societies—which means most of their people make their living in the valleys rather than the mountains. By contrast, mountain people make a living through raising livestock, mining, logging, smuggling, quasi-nomadic agriculture or other economic activities that take place in the mountains—not the valleys between them. Perhaps most important, most of these activities do not require them to defend a specific piece of terrain.

However the fact remains, there is no clear-cut, academic definition of mountain people. But, in the tradition of Supreme Court Justice Potter Stewart, you will know them when you see them. It will be even clearer if you choose to fight them.

The next obvious question is why are mountain people so hard to conquer? There is no question the difficult terrain favors the defenders—particularly if the defenders choose to fight as insurgents. While the invader can fight his way into the territory, he most often cannot locate and destroy the insurgents. The rugged terrain, with its caves, forests, cuts and gullies connected by routes known only to locals provides sanctuary and relatively safe movement even when outsiders have complete air supremacy. The mountains provide almost unlimited hiding places for local forces.

Further handicapping the invader, supply routes are always limited and frequently have numerous chokepoints which provide excellent locations for insurgent ambushes. To maintain a force in a mountainous area, the invader must provide security for long, vulnerable supply lines. He cannot concede the tactical advantage of the high ground to the insurgents but rarely has enough troops to control all the high ground on his lines of supply. YouTube has hundreds of videos of U.S. and allied forces blazing away at Afghan mountains hoping to suppress unseen insurgents. The invaders are usually firing blindly because despite owning the best technology in the world, they cannot find the insurgents. A few insurgents sniping at outposts and ambushing the supply routes tie down hundreds of government troops.

When dealing with outsiders, mountain people use the terrain to neutralize the attacker’s numerical or technological superiority—and their toughness to wear the interlopers down. When pushed by the Russians and then their Chechen puppets, the Chechens rebels withdrew to the mountains in the mid-1990s and continued their fight. In the last two decades, various Pakistani terrorist groups have maintained themselves in Khyber Pakhtunkhwa to continue their fights in Afghanistan and Pakistan. The Kurdish PKK has used its base in the mountainous southeast and east of Turkey to fight the Turks for over thirty years. And for over one thousand years, relatively small Christian communities in Lebanon and Syria have held their mountainous villages against lowland Muslim populations.

Campaigns against mountain people often become wars of attrition. In Afghanistan, the Soviets “successfully” invaded the Panjshir Valley nine times. Each time they caused great damage to the Afghan defenders, their families and their property. Yet each time, the Soviets withdrew and left the Afghan insurgents in command of the valley. Soviet and Afghan government forces were never able to suppress the insurgents and could not sustain forces in the rugged terrain. For while the Panjshir is a called a valley, it is really a long narrow corridor with limited access flanked by high mountains. In short, it is really a mountain fastness. Other than by rapidly depopulating a mountainous region, as Stalin did with the Caucasus in 1944, outsiders have required very long campaigns to integrate mountain people into their societies. The political will often failed and the outsiders withdrew, settling for a series of punishment raids to “discipline” and contain the mountain populations.

Even if an outside force does take control of a mountainous region, it will find it very difficult to maintain control. Unlike most lowland societies, mountain societies are physically fragmented, which leads to social fragmentation. While river valleys and plains provide natural lines of communication, which tend to unify a society, often by conquest, mountain ridges separate communities. In particularly rugged terrain, villages as little as ten miles apart by direct line can take a day or more to reach on foot. And during winter, they may not be able to visit each other at all. Just as important, mountain societies do not consistently produce the large surpluses necessary to support a bureaucratic government and thus have only infrequently been able to afford or need a central government to protect that surplus. In contrast, lowland societies have historically produced surpluses, have needed a government to protect those surpluses and developed the stratified social structures to do so. The presence of surpluses and lack of defensible terrain provided the incentive and the resources for strong men to unify these lowland regions. While most lowland societies become unified political entities, mountain societies usually remain fragmented. An invader must deal with each small political entity (family, clan, tribe, etc.) and with the long-term conflicts between them if the outsider hopes to control the mountain populations.