Why were the Mayas or Incas not sustainable? In the Thirty Years War of the 17th century, over 30 percent of the German population died. The Mayas and Incas did not have any wars with one tenth as many casualties. 

The Napoleonic Wars killed so many men that the Battle of Waterloo was fought with fourteen year olds. The wars in Europe till from the Thirty Years war in 1618 to WW2 in 1945 were the bloodiest in history.