Polynesian society was theocratic in structure. Chiefs that were believed to be descendents of the gods, also held the position of high priest. The high priest would mediate between god and man to win the god's favor, and get food and supplies for the people. This power of mediation was known as mana. The mana entitled him to sacred rights (tapu, or taboo) over land, fishing grounds, and more. On most islands settled by the Polynesians, these two factors held society in balance, and carefully regulated relations between people and the chief.
The Polynesians had found the island around the third century AD, and brought with them sweet potatoes, bananas, and sugar cane. Although with only a triangle of extinct volcanoes roughly seventy square miles in extent to work with, they harvested the land, and eventually raised a population of over 7000 people. Over the next 700 years, they managed to carve and raise more than 300 giant statues, generally about five times life-size, on extensive temple platforms. During the 16th century, the islanders also invented a script, which it is thought was used by priests to help memorize oral traditions and genealogies.
Then things started to go wrong. The growing population "denuded the island's environment. Forest clearance reduced rainfall, and the fields yielded less; it also reduced the yield of timber from which canoes were built, thus diminishing the harvest of the sea." A new flaked obsidian spearhead called a mata'a appeared on the scene, along with a new warrior known as "'tangata rima toto'-the men with the bloodied hands." Two separate groups formed, and separate ends of the island were in a state of constant warfare. The chief began to lose his power, and his mana was no longer respected. Over the course of the war, the statues were systematically toppled, either to insult the mana of an enemy clan, or as a sign of rebellion by commoners against the chiefs whose mana had failed them. The theocratic state was replaced by a new system of government: "the men with the bloodied hands" competed to be the first to find an egg of the sooty tern (a type of bird) thus winning chieftainship-for a single year.
Dutch voyager Roggeveen landed on Easter Island in 1722, and found a state of anarchy. By the end of the 19th century, their degeneration-"compounded by European slave-raiding and the diseases the Europeans had introduced"-had reduced the population to 111 people, who barely retained even the sketchiest oral traditions of their past. From their stories, as well as archaelogical evidence, historians put together a picture of what was known as the Decadent Phase. It showed endemic warfare, as well as possible signs of cannibalism.
*This information was derived from "A History of Warfare" by John Keegan