Nationhood and Independence
The Igbo people were up against two separate kinds of disenfranchisement. First, they are the victims of the historical "Rape of Africa" by Europe, during which European nations "settled" Africa, but only to exploit Africa of its natural resources. This feud is settled when the Nigerians declare their independence in 1960. But, there is another layer of disenfranchisement: The Igbo people are regarded hostilely in Nigeria, except in Biafra where they are the political majority.
Warfare
When the Biafran territory declares its political independence from Nigeria, Nigerian rejects the legitimacy of their independence. The two nations are launched into civil war, one of the most historically brutal and cruel wars in human history (though there are many, many horrific wars). The theme of warfare is not uniquely Nigerian, but for some reason, the story is not that well known in the West, so this book attempts to recount the horrific truth of political strife, a serious object of human history that should be taken seriously as history. This thematic focus is unenjoyable, but that's precisely Achebe's point in telling the story.
Art, family, and meaning
War is not what defines Nigeria, though, because Achebe himself is part of Nigeria's heritage. Achebe's life is a direct rejection of African stereotypes. He was a young political man with a serious passion for art, love, and human freedom. In other words, he is deeply human as a person, so the inclusion of his story serves to stave off bad understandings of African politics. Achebe's personal life is a quest for meaning. When he has the choice to fulfill his dreams by going to school for creative writing, he chooses to marry his wife and build a home and family with her. Then he writes anyway and ends up famous and successful. He found meaning and success by sacrificing his desires for the betterment of his family. This is an example of life, well-lived.