The dynasty that built the empire began in 127 BC in the Yarlung Valley, south of Lhasa. For centuries the Yarlung kings ruled a modest territory, but around 618, Namri Songtsen consolidated power over neighboring clans before being assassinated. His son Songtsen Gampo inherited the consolidation and transformed it into conquest. He subjugated the Sumpa, annexed the Zhangzhung kingdom after a three-year campaign beginning in 642, and defeated the Tuyuhun who controlled trade routes around Lake Koko Nur. When the Tang emperor refused his request for a Chinese princess in marriage, Songtsen Gampo threatened Songzhou with an army of over 100,000 men. The emperor relented. Princess Wencheng arrived in 640, and Buddhism, which Songtsen Gampo is traditionally credited with introducing to Tibet, began its long transformation of the plateau.

After Songtsen Gampo's death in 650, the empire did not contract -- it accelerated. The Tuyuhun buffer state was fully absorbed by 663. In 670, Tibetan forces won the Battle of Dafeichuan and seized the Chinese Four Garrisons of Anxi in the Tarim Basin, cutting off Tang control of the Silk Road for over two decades. The Gar clan of ministers wielded enormous power during this period, until Emperor Tridu Songtsen lured the entire clan -- more than 2,000 people -- to a hunting party in 698 and had them massacred. It was a brutal assertion of royal authority, and it worked. By the mid-8th century, under Trisong Detsen, the empire reached its greatest power. While China reeled from the An Lushan Rebellion beginning in 755, Tibetan armies pushed east, capturing Chang'an itself in 763.

The Tibetan military was distinctive. Soldiers wore lamellar armor and chainmail, proficient with swords and lances though less skilled as archers than their steppe neighbors. Officers were part-time warriors designated by a golden arrow seven inches long, gathering annually to swear fealty and every three years for a sacrificial feast. On campaign, Tibetan armies carried no grain and lived off plunder -- a logistics strategy that rewarded speed and aggression. Their armor was prized across Central Asia; when the Turgesh khagan Suluk fought the Arabs, he reportedly wore Tibetan armor that stopped two arrows before a third penetrated his chest. The empire's reach extended to wars with the Abbasid Caliphate, with Tibetan forces active as far west as Samarkand and Kabul.

Two stone pillars in Lhasa record the empire's diplomatic history in physical form. The Zhol Pillar, erected around 764 at the base of the Potala Palace, bears the oldest known Tibetan inscriptions and documents military campaigns against China, including the capture of Chang'an. Outside the Jokhang Temple stands the pillar inscribed with the Sino-Tibetan treaty of 821-822, a bilingual agreement that established borders and peace between Tibet and Tang China. Under Emperor Ralpachen, who negotiated this treaty, Tibet reached its greatest territorial extent, stretching from Mongolia to Bengal. Ralpachen was also one of the three Dharma Kings credited with establishing Buddhism in Tibet, and he commissioned the Mahavyutpatti, a Sanskrit-Tibetan lexicon of thousands of terms that standardized Buddhist translation.

Ralpachen was murdered in 838 by supporters of his brother Langdarma, whose brief reign saw the Uyghur state collapse to the north, flooding Tibet with displaced peoples. When a Buddhist hermit assassinated Langdarma in 842, the succession dispute that followed shattered the empire. The period that came after, known as the Era of Fragmentation, saw Tibet splinter into competing warlords and regional powers with no central authority. The empire that had rivaled Tang China and fought the Abbasid Caliphate dissolved in less than a generation. But its legacy endured in the institutions it created -- the written script, the Buddhist establishment, the diplomatic traditions -- and in the stone pillars that still stand in Lhasa, recording a time when Tibet treated the mightiest empires of Asia as equals.