The Battle of the Windmill represents World War II.
Russia may have been on the winning side of World War II, but they lost huge numbers of citizens—up to 11 million soldiers in the war, and maybe even more civilian casualties. The low point came in December 1942, when the German army pushed within twenty miles of Moscow. The Soviets managed to push the Germans back and protect Hitler's next goal—their southern oil fields—but only with a lot of death and destruction.
The worst may have been over by the time Orwell was wrapping up Animal Farm, but the Soviets were on the offensive from April 1943 until the war ended two years later.
Animal Farm has its own miniature version of World War II in the Battle of the Windmill. Frederick's men advance, take a pasture and blow up the Windmill. As the enemy rushes onto the farm, "even Napoleon seemed at a loss" (8.16). A message arrives from Pilkington telling Napoleon, "Serves you right" (8.16).
After some super violent fighting, which includes Boxer using his hoofs to smash in the heads of the men, the animals end up winning but being "weary and bleeding" (8.23). Almost immediately, Squealer begins proclaiming the war as a proud victory for Napoleon—but even dumb old Boxer doesn't quite buy it. When Squealer points out that they have regained the farm, all Boxer can say is, "Then we have won back what we had before" (8.31).
No matter how good your ministry of propaganda, it's hard to spin a war in which millions of lives are lost.