in spite of the efforts of Napoleon andBoxer to rally them, they were soon driven back. A number of them were already wounded. They took refuge in the farm buildings and peeped cautiously out from chinks and knot-holes. The whole of the big pasture, including the windmill, was in the hands of the enemy. For the moment even Napoleon seemed at a loss.

The “fifteen men” represent the overwhelming force that Hitler threw against the Soviets. The fact that the animals are “soon driven back” represents the great success the Germans initially experienced. The fact that the animals feel overwhelmed represents the way in which the Soviets also initially were overwhelmed by the German onslaught. The Germans quickly seized a huge swath of Soviet territory, just as Frederick’s men seize the “whole of the big pasture, including the windmill.” Finally, the fact that Napoleon seems “at a loss” resembles the way in which Stalin felt shocked and dumbfounded by the Nazi surprise attack.

The destruction of the windmill represents the ways the Nazis destroyed much of the industrial infrastructure of the Soviet Union.

In short, Orwell’s account resembles, almost point-by-point, real historical events in the early 1940s.

As one reads this allegory of George Orwell, it is important to keep in mind that the author is really telling the story of Communism. With this ideology, there were those like Leon Trotsky, represented by Snowball, who were brilliant and sincere about improving conditions for workers, while others such as Josef Stalin, represented by Napoleon, were more concerned about political advancement and changes in government and their personal power.

Under the direction of Snowball, then, the animals begin construction of a windmill which will provide energy: heat in the winter and electricity to run some machines which will ease the burden on the animals.

Snowball conjured up pictures of fantastic machines which would do their work for them while they grazed at their ease in he fields or improved their minds with reading and conversation.

Thus, the windmill is symbolic of modernization and progress from the "peasantry" of the animals whose manual labor is all that runs the farm. Communist leaders strongly felt that electricity was essential for their new nation as the infrastructure of the Czars was yet feudal and Russia was not keeping up with the West. Snowball, like Trotsky, yearned for Russian progress.

You can find the answer to your question in chapter 7. When the windmill collapses, Napoleon immediately blames Snowball, claiming that footprints leading to a hole in the hedge were undoubtedly his. The humans, however, don't believe in Snowball's guilt. According to the narrator, "Out of spite, the human beings pretended not to believe that it was
Snowball who had destroyed the windmill." They give an alternative reason for the collapse, but you're going to have to find that cause for yourself in chapter 7. Even though the animals are convinced that the humans can't be right, they decide to rebuild the windmills walls almost twice as thick as they had been.

I've pasted a link to an online text of the book so that you can easily find the answer. Hint: Look at the second paragraph.