Which act destroys the symbol of all of the animals' work, but unites the animals again?
In Chapter 8, the farm is invaded by a group of hostile humans, who blow up the windmill that the animals had just finished building. In the ensuing battle, many animals and some humans are killed, but the animals succeed in driving the humans off the farm. Despite the many dead and injured animals, the Pigs, especially Squealer, consider the battle a victory.
The windmill represented all of the animal's work; it was the central focus of all their efforts and the physical representation of the better life that their new society was going to afford them. Its destruction represents how weak that society actually was.
However, the animals are again united by the propaganda show that the pigs put on in order to stifle any criticism in the aftermath of the battle. They declare that they can build six windmills if they want to; their victory is proof of their superiority in the face of adversity and their will to persevere. As so many times before, the animals are essentially hypnotized by a parade of songs and speeches that carefully distract them from reality.