Baghdad
The first systematic translation civilization.
Kernel
Founded as the Abbasid capital in 762 on a circular plan, Baghdad became, within a century, the largest city in the world (~1 million people) and the operating site of the Bayt al-Hikma (House of Wisdom) — a state-funded translation project that rendered Greek, Persian, Indian, and Syriac scientific works into Arabic. The mathematical, astronomical, and medical knowledge re-exported westward to Europe over the next four centuries is mostly Baghdad's work, not Greece's directly.
Why it rose
Geography at the head of navigable Tigris commerce. Caliphal patronage. Paper, recently arrived from China via the 751 CE Battle of Talas, replaced parchment and dramatically reduced the cost of intellectual reproduction. Add an explicit civilizational ideology — that retrieving and translating foreign knowledge is a religious good — and a research city follows.
What it gave the world
Algebra (al-Khwarizmi, ninth century). The Hindu-Arabic numeral system that the West still uses. Optics (Ibn al-Haytham). Medical canons (al-Razi, Ibn Sina) that taught European doctors for five centuries. The translation infrastructure that re-introduced Aristotle to medieval Europe via Latin translations from Arabic, made in Toledo from Baghdad's source texts.
Why it declined
Internal fragmentation through the tenth century. The 1055 Seljuk takeover. The 1258 Mongol sack — Hülegü's siege killed perhaps 200,000 people and threw the libraries into the Tigris until, by report, the river ran black with ink for a week. The center shifted; Cairo and Damascus partially absorbed the cultural function. The institutional damage was permanent.