The statement that best expresses the theme of the passage is: It is important to recognize a dictatorship before it is too late.
In the passage, the animals' desperate attempts to save Boxer highlight their ignorance of the true nature of the leadership on the farm. Boxer’s fate serves as a symbol of how individuals can be exploited by a corrupt regime, and the urgency in the animals’ cries underscores the tragedy of not recognizing the perilous situation until it is too late.