In chapter 8 of Animal Farm, when told that the Battle of the Windmill was a victory, explain what Boxer says and what it means.

Boxer does not agree at all with Squealer's contention that the battle had been a victory, for he asks him 'What victory?' Boxer is injured after the battle: his knees are bleeding, he has a split hoof and a dozen pellets are buried in his hind leg. Squealer, who is clearly irritated and surprised by Boxer's cynicism, tells him that their victory lies in the fact that they have successfully driven the enemy (Frederick and his men) off the farm, which he calls 'sacred soil.' 

Boxer, however, does not relent. He tells Squealer:

"But they have destroyed the windmill. And we had worked on it for two years!"

He is clearly upset and cannot fathom that the wanton destruction of their terribly hard work can possibly be equated with a victory. 

Squealer's response is quite emphatic and it is clear that he wants Boxer to not argue the point and wishes him to shut up and stop asking uncomfortable questions or saying something inappropriate, but Boxer insists on having the final say in the matter and says:

"Then we have won back what we had before." 

Squealer ends their verbal tit for tat by saying that what Boxer has just said is their victory. 

This is the second time that Boxer has actually opposed the pigs and their perception of events. The first was when he challenged their version of events about Snowball being a traitor from the outset.

"I do not believe that Snowball was a traitor at the beginning," he said finally. "What he has done since is different. But I believe that at the Battle of the Cowshed he was a good comrade."

Snowball was quite upset about Boxer's reaction and told him that Napoleon stated categorically that Snowball had been Jones' agent from the very beginning, even before the Rebellion. Boxer then accepted what was said stating that "Napoleon is always right" and adopting this as another maxim.

Soon after, Boxer was attacked by three of Napoleon's dogs during his purge in which so many animals confessed and were slaughtered, but Boxer fought them off and almost killed one of them.

Boxer has always been accepting of the pigs' version of events up to these two incidents. His response in this second instance is as significant as the previous one in that it shows that he has the ability to stand up to the pigs and question their manipulation and propaganda. As such, he could become a very powerful leader and a force against the pigs.

It soon becomes clear that Boxer's confrontation with Squealer is deemed a threat. The pigs recognize the respect and admiration he receives from all the other animals and know that they would follow him in any action, including fighting Napoleon's dictatorship. It is for this reason that we later read of the contemptible and deceitful manner in which the pigs finally get rid of him.

When Boxer is seriously ill after a hard day's labor, the pigs supposedly call for the veterinarian to take him to the hospital. They, however, sell him to the knacker and have the temerity to buy themselves a case of whiskey with the proceeds.