The Tang dynasty, and especially the Song dynasty that followed it, presided over roughly six centuries of medieval Chinese history during which China developed one of the most economically and technologically advanced societies anywhere in the medieval world. Paper money, movable-type printing, gunpowder weapons and magnetic compass navigation were all developed and in genuine practical use under the Song, well before comparable technology reached Europe, even as the same dynasty's military structure ultimately proved unable to stop the Mongol conquest that ended it and founded the Yuan dynasty in its place.
The Song dynasty built a genuinely advanced economy well before Europe caught up
Song China issued the world's first government-backed paper currency, developed movable-type printing centuries before Gutenberg's press, refined gunpowder into practical military weapons, and used the magnetic compass for maritime navigation. That technological base sat on top of a large, urbanised, commercially sophisticated economy, extensive canal networks moved goods between cities, and a professional civil bureaucracy, selected through competitive examination rather than hereditary privilege, administered the whole system.
None of that economic and technological lead stopped the Mongol conquest
The Song dynasty's own governing philosophy, shaped partly in reaction to the military warlordism that had destabilised earlier periods, deliberately emphasised civil administration over military power, leaving its armies structurally weaker relative to its economic and administrative sophistication. The Mongols under Kublai Khan conquered the Southern Song by 1279, founding the Yuan dynasty as the first foreign, non-Han dynasty to rule the whole of China, and folded Chinese territory into the wider Mongol trading network that briefly connected China to Europe, the same network Marco Polo travelled a generation later.
What we're still unsure about
That the Song dynasty achieved genuinely advanced economic and technological development, and that the Mongols conquered it despite that advantage, are well established, extensively documented facts of medieval Chinese history. What's more genuinely a matter of ongoing historical debate is exactly how much weight to put on the Song's deliberate civil-over-military policy choices, as opposed to genuine Mongol military innovation, composite bows, exceptional cavalry mobility, adopted siege technology, in explaining the conquest's outcome, and historians continue to argue over that relative causal balance rather than treating it as settled.
This sits inside Medieval China: Tang, Song & Yuan Dynasties, one of eight topics in Medieval History, one of seven domains in History, one of seventeen subjects the app can quiz you on.