Why and in what way does China Achebe present society in Things Fall Apart?

Achebe's description of Igbo society paints a picture of a fully functional system in which inhabitants are comfortable with what they have. It is clear that the Igbo have an extended history and their way of life has been developed over a substantial period of time. He describes their traditions, laws, customs, religious beliefs, laws and the hierarchy existent within their society. It is evident from his explanations that, just as in any society, the majority obey its rules and meet its requirements but that there are also others who are rebellious and do, at times, oppose laws and tradition.
The Igbo have, in this regard, rules, laws and traditions by which those who display opposition, recalcitrance or disrespect are sanctioned. It was, for example, so with Okonkwo when he assaulted his wife during The Week of Peace. He was severely sanctioned and paid a hefty price for his disobedience. On another occasion, he accidentally killed a youngster during a funeral. The punishment here, even though it was an accident, was harsh. Okonkwo was banished to his mother's village for seven years.
Within this structure, Igbo society was generally peaceful. Some laws and traditions were extremely harsh, such as the punishment for twins who, at birth,  are banished to and abandoned in the Evil Forest. Disputes between villages were normally resolved amicably and were the result of complicated negotiations. The purpose of these was to ensure fairness to all parties involved and so it was when Ikemefuna, from another village, was placed in Okonkwo's custody as compensation for the murder of a citizen. The boy's life was later taken with Okonkwo being the executioner.
The presentation of Igbo society in this way is to contrast its generally peaceful existence with that which was introduced by the British colonialists after their arrival. The imperialists introduced new laws and a new religion which was in opposition to those of the Igbo. The imposition of these created conflict not only against the invaders, but also between members within Igbo society. Some members of Igbo society (such as Okonkwo) most vehemently resisted the colonialists' influence, whilst others adopted their heritage, as Okonkwo's son, Nwoye, did.
The obvious point that Achebe makes is that the introduction of a foreign power and the violent imposition of its laws and religion, is what leads to the eventual destruction of native heritage. Once a foreign culture makes its presence felt and imposes itself by severely punishing those who fight it, the indigenous culture does not stand a chance. Igbo customs, for example, were eroded and created internal strife. Members became enemies of one another and that sounded the death knell for Igbo society. In the end, its own culture had been corrupted, destroyed and then replaced by that of the imperialist.