What is ironic about their victory celebration?

Rebuilt completely, the windmill is once again destroyed, this time by Frederick and his followers who try to retake Animal Farm, but are defeated, inflicting many casualties on both sides. To celebrate their victory, the pigs get drunk off a case of whiskey found in the cellar of the farmhouse. A few days later, the animals realize that they have remembered another Commandment incorrectly. It now read: "No animal shall drink alcohol to excess." With so little opposition to this outright alteration of fact, nothing stands in the way of the pigs.