The deadliest army on earth can't top the weather for its destructive potential. History's mightiest empires have fallen for no more of a reason than climate change leading to failed harvests and a starving population.

But you wouldn't know that from most stories of the past. History was long about diplomatic treaties, battle tactics, or the biographies of great leaders. In the last 40 years researchers have increasingly compensated by looking at climate as a major factor in the course of human civilization. They have analyzed the history of weather patterns, climate change, ocean currents, and even geology to explain why some societies thrive and others die.

In this episode we look at four civilizations that were destroyed or permanently crippled by changes in the weather. They includeThe Neo-Assyrian Empire, which was destroyed by a joint Babylonian and Median attack in 612 BC, but initially crippled by a severe drought that was so bad a priest commented that “no harvest was reaped” one year The Greenland Vikings, a one-thriving trade empire that stretched from Denmark to Newfoundland and delivered walrus tusk to medieval royalty but dissapeared suddenly in the 1400sMedieval Iran—the economic powerhouse of early Islam that grew enough cotton to change the fashion tastes of the Middle East...until a cold snap killed whole crops, leaving it weakened until the Mongols finished the job in the 1200sEurasia of the 1500s, which due to the Little Ice Age saw massive revolts, from the 30 Years War between Protestants and Catholics to the Jelali Revolts of the Ottoman Empire to civil unrest in India and China