What was the most significant theme in Achebe's Things Fall Apart?

While there are several significant themes in
Things Fall Apart, such as individual alienation and loneliness,
good and evil, clash between cultures, and custom versus tradition, one might say that
the most significant is the theme of choice and consequence because it embraces the
others. This theme is demonstrated from the early years of Okonkwo's life when he
rejects his father's legacy and defeats the chi that links them together in the same
pattern. It is still seen as the driving force in Okonkwo's life just before it ends
after he decapitates a man.

Okonkwo's choice to challenge
his early chi leads to the consequence of alienating and mistreating his son Nwoye and
to being demanding of and unkind toward his wives. His choice to participate--against
reason and against wise advice--in the ritualistic sacrificial death of Ikemefuna leads
to the consequences of a completely shattered relationship with his son, a shattering
which may be shown metaphorically in the descent of the
locust:

Mighty
tree branches broke away under them, and the whole country became the brown-earth color
of the vast, hungry
swarm.

There are many other
instances of choices with potent consequences in Okonkwo's life but the ultimate choice
with the most final consequences is his choice to fight against the missionaries and
British officials. His choice to decapitate an official leads to a double consequence.
The first consequence is that he learns his tribe has changed so deeply during his exile
that they do not now stand behind his act of vengeance. The second consequence is that
he sees that since his tribe has stood against him, he ironically has no choice left but
to enact an ignoble and dishonorable end for himself, and so he takes his own
life.

The
white man is very clever. He came quietly and peaceably with his religion. We were
amused at his foolishness and allowed him to stay. Now he has won our brothers, and our
clan can no longer act like one. He has put a knife on the things that held us together
and we have fallen apart.