Chinua Achebe himself, as many critics show, did not portray the conflict as between the sides of good and bad - that the white man was evil whereas the African's were goodness personified. He showed a balance in his novel which tackles both sides of the questions with a maturity lacking in many novels dealing with similar issues. In fact, the question of imperialism isn't a major issue until much later in the novel. The novel deals most importantly with the culture and the slow change brewing both within it, and from the effects of the outside. The book is indeed a masterpiece.
The story centers around Okonkwo. From his childhood he has suffered from the shame of having a lazy father. To compensate for his father's cowardice, he shuns all that his father stood for including gentleness, and love. From an early age, working hard to grow his own farm, he shows prowess in war and wrestling, and is harsh towards all love and warmth. His whole family stands in awe of him knowing that he will not hesitate to beat anyone for disobedience. His oldest son Nwoye fears him while trying to live up to his father's expectations.
Despite it all, Okonkwo is not as brave as he desires to be. He fears to be thought weak and this fear is what leads to his every action. It is paradoxically what led to his success. The Ibo culture values the qualities of manliness and hard work in a man and it is those very qualities that Okonkwo nurtures to act against his father's weakness and laziness. He is a self-made man. But his downfall is inevitable. His fear had gone too deep.
Okonkwo spirals towards self-destruction in a world of his own creating; in which he believes that gentleness and love are not qualities to be admired. After accidentally killing a young boy, in accordance to custom, he is banished from his village for seven years. In those seven years, the whole village of Umuofia changes. The white man has come and with him, his own ideas. Ideas which threaten to squash Okonkwo's own world. Having no idea of anything outside harshness and brutal strength, Okonkwo has gone further from the ideals of his people than he realizes. His people are not willing to fight for their ways and Okonkwo feels that they have become a 'womanly' clan. In desperation, he acts according to his perception of his clan's manliness.
The story deals with the concept of change. Has the society of Umuofia changed or is it Okonkwo who is different? Did the white man bring about the change in Okonkwo's people, or was the culture of Umuofia resilient to change? adapting the new culture within its own? Okonkwo may be a product of change, or of his own world. But either way, he himself is unable to change and his narrow perspective of looking at things inevitably leads to his doom. These questions are tackled by various critics and their really isn't any one answer. In the end, the answer lies within each person's own perception of what change really is: the Ibo people adapting to the new culture, or Okonkwo struggling to maintain his idea of a culture that never really was.