When has Afghanistan had a stable society that didn't define itself with tribal and/or cross border conflicts?
I don’t think Afghan society has ever defined itself by conflict (brief outline of my understanding of Afghan history follows…)
Tribalism is an important element to consider, but it’s not as simple as dismissing the entire society as inherently fractious. For example, the Pashtuns of the east/south and Uzbeks of the north may both be described as “tribal”, but their structures/hierarchies of authority are of a different nature (the former being more egalitarian, the latter more authoritarian). The Tajiks are not really tribal in any significant way as far as I know (I’m not sure about the Hazaras).
In terms of cross-border conflicts, I’m not entirely sure what you’re referring to. The modern Afghan state has its origins in the mid-18th century Durrani Empire, which was a lot bigger back then, encompassing parts of modern-day north-east Iran and Pakistan/India as well as Afghanistan (it was itself built on the ruins of the Safavid, Mughal, and Uzbek Empires which had divided modern-day Afghanistan between them previously). It shrunk due to the encroachments of others - the Qajars in the west, and the Sikhs and British in the east. The one remaining territorial claim that Afghanistan makes on its neighbors is to the Pashtun tribal areas of the old North-West Frontier Province, now Khyber-Pakhtunkhwa in Pakistan. But the cross-border tensions there are the product of a border formalized and consolidated by the British in the late 19th century.
And it should also be noted that there have been no significant separatist movements in modern Afghanistan, despite the presence of shared ethnicities across borders. Up to this day, all tribes, ethnicities, and sects have remained committed to the endurance of Afghanistan.
In terms of civil unrest, there seems nothing especially uniquely troubling about Afghanistan. From the late-18th century until the first Anglo-Afghan War (1839-1842), there were regular dynastic struggles for power among the Durrani elite, similar to the succession wars that plagued others such as the Ottomans and Mughals. But the general population were not involved. They only began to play a role in the politics of the state in response to the British interventions of 1839-1842 and 1878-1880 (second Anglo-Afghan War), and only then around Kabul and Kandahar. In this period, Afghanistan was de-centralized, with the capital (first Kandahar, then Kabul) barely able to exert authority over the other regional centres (Herat in the West, Kandahar in the south, Balkh/Mazar in the north, Jalalabad to the east).
After 1880 Abdur Rahman centralized the state for the first time, giving Kabul the means to directly intervene in provincial affairs and subjugate the population. But it was only when Kabul used these means to try to force social and economic changes on conservative tribal areas that limited civil strife resulted (most obviously in the brief civil war of 1929 which overthrew Amanullah).
From 1929 to 1978, Afghanistan was largely peaceful, two or three minor episodes aside (the coup of 1973 was bloodless). The rulers very cautiously began opening up the country to Soviet and American aid after WW2, and especially during the 60s. The result of this measured modernization program was to disrupt traditional structures of authority (in the household, in the tribe, across the country, etc.) and to empower groups previously excluded from politics. In Kabul University in the 60s and 70s, this process produced two broadly-defined groups seeking revolutionary change - the communists and Islamists. The former seized power in a coup in 1978, and (with Soviet backing) set about implementing an extremely far-reaching and repressive socialist program which immediately alienated most of the population and provoked the first country-wide uprising in response. The latter finally took Kabul in 1992 after taking advantage of Cold War and regional politics to dominate the uprising with American, Pakistani and Saudi aid. By then, the damage was done. But even in this, there is nothing unique - the destabilizing impact of a modernizing state seeking to bring a previously closed society into the twentieth century while navigating late 20th century geopolitics can also be observed in places like Ethiopia and the Yemens (off the top of my head).
So I think it’s fair to say that the conflicts plaguing Afghanistan today are the product of a number of forces, of which tribal and ethnic tensions are just two, and certainly insufficient to explain things.
