Following the animals' great victory over the humans at the Battle of the Windmill, a case of whiskey was discovered in the cellar of the farmhouse. The pigs decided to celebrate and all of them became quite drunk. Comrade Napoleon was not actually dying; he was only drunk. When Squealer was later found "sprawling," it was because he had broken the ladder while secretly making changes to the Commandments. Much fatter now from stuffing himself on human luxuries, Squealer's weight was too much for the ladder. Old Benjamin seemed to realize what was going on even if the other animals did not, and Muriel appeared to be the only other animal to realize that the Fifth Commandment had been amended: Alcohol consumption was now OK as long as it did not lead "to excess."
Despite the commandment prohibiting alcohol consumption, in chapter eight the pigs have gotten drunk and hungover. Because farmer Jones's alcoholism had led to the animals being neglected before they took over the farm, they have decided not to drink--until the pigs break the rule.
In chapter eight, Frederick and his men use dynamite to blow up the windmill. The angered animals drive them away, though some of the animals are killed in combat. The pigs declare the battle a "victory" even though the windmill, which cost them so much to build, has been completely destroyed. While at first the bleeding animals don't see much to celebrate, the pigs convince them they have won a great victory.
At the end of chapter eight, in a darkly comic last section, the pigs find a case of whisky in the farmhouse cellar. They drink it, accounting for the hangovers that leave them feeling "dying" and "stunned." Napoleon, thinking he is dying, decides that alcohol consumption will be punished by death. Then, as the pigs recover with no lasting ill effects, Napoleon decides to plant barley and learn how to make whiskey. Squealer changes the fifth command, which originally decreed that no animal would drink, by adding the words "to excess" at the end.
These last paragraphs serve to illustrate how self-serving the pigs are, changing the rules to suit themselves, and how easily the other animals are deceived.