The statement that best expresses the theme of the passage is: It is important to recognize a dictatorship before it is too late.
This reflects the urgency and desperation of the animals who realize that Boxer is being taken away to his death, highlighting their inability to act in time to save him. The passage illustrates the consequences of ignorance and the failure to recognize the oppression the animals are under, which aligns with the idea of recognizing a dictatorship before it leads to tragedy.