The pass begins at Kadam near Jamrud, 18 kilometers west of Peshawar, and climbs to its summit at Landi Kotal, five kilometers inside Pakistan. A lesser section continues another eight miles westward to the Afghan town of Loya Daka. Along this route, which now follows Asian Highway 1, silk, jade, rhubarb, ivory, pepper, and textiles once moved between China and the Mediterranean. The Parthian Empire fought for control of passes like this one to profit from Silk Road commerce. Through the Khyber, the ancient kingdom of Gandhara -- in present-day Pakistan -- became a regional trade hub connecting Bagram in Afghanistan to Taxila in India. It was not merely goods that traveled this road. Buddhism spread eastward through here. Islam came through heading the same direction. Armies of the Saka, the Yuezhi, and Genghis Khan's successors all funneled through this gap. The geography made it inevitable: mountains to the north, mountains to the south, and one navigable corridor between them.

To the north of the pass lies the territory of the Shalmani and Mullagori tribes. To the south stretches Afridi Tirah. The villages within the pass itself belong to Afridi clansmen, and for centuries the Pashtun clans have treated it as their personal preserve. Safe conduct came at a price -- always. This was their primary income, and they defended it accordingly. The Islamic period brought a new parade of conquerors: Mahmud Ghaznavi, Muhammad of Ghor, Timur, Babur, and Nader Shah all used the Khyber and nearby passes to invade the subcontinent. During the British era, the pass became the theater for some of the most storied episodes of colonial warfare. Jamrud Fort, at the pass's mouth, changed hands repeatedly. The British Indian Army once marched an elephant battery of heavy artillery through the defile. During World War II, the British erected concrete dragon's teeth on the valley floor, fearing an Axis invasion of India that never came.

When India was partitioned in 1947, the Khyber Pass fell to Pakistan, and the Durand Line -- drawn by a British diplomat in 1893 -- became the international border it bisects. The pass gained new strategic significance during the Soviet-Afghan War and again after September 11, 2001, when it became a critical NATO supply corridor into Afghanistan. That role made it a target. In 2010, after a NATO helicopter attack accidentally killed three Pakistani soldiers near the border, Pakistan closed the pass to supply trucks. A convoy of NATO vehicles queued at the checkpoint was attacked by insurgents, who destroyed more than 29 oil tankers and trucks. The incident forced the U.S. and NATO to broaden their supply routes through Central Asian nations. Even the Iranian port of Chabahar was considered as an alternative -- a measure of how irreplaceable the Khyber had been.

The Khyber Pass has lodged itself so deeply in global consciousness that its name appears on roads, pubs, and hiking trails across four continents. There is a Khyber Pass Road in Auckland, New Zealand, and a Khyber Road in Dublin's Phoenix Park. Glasgow has a steep, twisting road named for it in Mugdock Country Park. Philadelphia has a Khyber Pass Pub. Before the partition of India, a common Hindustani phrase described the length of the subcontinent as 'Khyber se Kanyakumari' -- from the Khyber to India's southern tip. Rudyard Kipling wrote of it. A railway once climbed through the pass in a series of dramatic switchbacks, operated intermittently by Pakistan Railways and briefly revived as the Khyber Steam Safari in the 1990s. The pass remains what it has been for millennia: not just a geographic feature but a symbol of the threshold between worlds, the point where Central Asia ends and the Indian subcontinent begins.