The rise of Genghis Khan and the vast Mongol empire was helped by a temporary spell of good weather, researchers have claimed.
Studying the rings of ancient trees in central Mongolia, the academics believe they may have discovered how the group of nomadic horsemen managed to conquer much of the world.
The research found that exactly when the empire rose, the normally cold region of central Asia saw its mildest, wettest weather in more than 1,000 years.
This caused grass production to boom and as a result so did vast numbers of war horses and other livestock, which gave the Mongols their power.
Amy Hessl, co-author with Neil Pederson from Columbia University's Lamont-Doherty Earth Observatory, said: "The transition from extreme drought to extreme moisture right then strongly suggests that climate played a role in human events."
"It wasn't the only thing, but it must have created the ideal conditions for a charismatic leader to emerge out of the chaos, develop an army and concentrate power.
"Where it's arid, unusual moisture creates unusual plant productivity, and that translates into horsepower. Genghis was literally able to ride that wave."
Prior to this the Mongol tribes were racked by disarray and internal warfare, but this ended with the sudden rise of Khan in the early 1200s.
He united the tribes into an efficient military state that rapidly invaded its neighbours and expanded, soon ruling most of what would become modern Korea, China, Russia, eastern Europe, southeast Asia, Persia and India.
The researchers also said the tree rings gave them an indication of what the future could hold for the region, because as the world warms, drought and other extreme weather could become more common in Asia.
"This last big drought is an example of what may happen in the future, not just in Mongolia but in a lot of inner Asia," said Mr Pederson.
"The heat is a double whammy - even if rainfall doesn't change, the landscape is going to get drier."
Other historical events that studies say were affected by climate include the disappearance of the Maya, the expansion and fall of Roman imperial power and the 13th-century collapse of south east Asia's Angkor civilisation.