Russia suffered enormous casualties in World War II. It is estimated that the Soviet Union lost roughly 11 million soldiers in the war, along with perhaps even more civilian casualties (source).
In December of 1942, the German army had pushed within twenty miles of Moscow. It was only through a massive counter-offensive that the Soviets managed to push the Germans some forty to fifty miles back. Hitler then shifted his strategy and began aiming at oil fields in the southern Soviet Union. The Soviets were able to thwart the plan, but only with many more losses.
By the time Orwell was wrapping up Animal Farm, the war was not yet over, but, at least for the Soviets, the worst had passed. From 1943 on, the Soviets remained on the offensive until the end of the war in April 1945.
Animal Farm has its own miniature version of World War II in the Battle of the Windmill. Things begin rapidly as 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).
For the fable genre, the fighting in Animal Farm is extremely violent. A number of animals are killed, and Boxer uses his hoofs to smash in the heads of the men. Though the animals end up winning, they find that they are "weary and bleeding" (8.23). Almost immediately, Squealer begins proclaiming the war as a proud victory for Napoleon.
Boxer the horse, like so many that survived the war, no longer understands the word victory. 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. Even the loyal Boxer understands that.