The question refers to The Battle of The Windmill and to correctly respond to the question, one has to understand the circumstances and the result of the battle. After Frederick had cheated the pigs by giving them counterfeit notes to pay for the wood he had purchased, Napoleon passed a death sentence on him and warned the animals that they should be ready for the worst.
The next morning, Frederick and his men launched an attack. Although the animals were prepared, they did not achieve the easy victory which they had had in the Battle of the Cowshed. This time, the men were well-armed and as soon as they came within range of the animals, they opened fire and drove them back, despite Napoleon and Boxer's efforts to encourage them. The animals hid in the farm buildings and watched the men. Napoleon was at a loss and hoped that Pilkington would come to help, but that hope was lost when the pigeons returned with a cynical message from the farmer.
The humans had gathered around the windmill and the animals were dismayed. When two of the men produced a crowbar and sledgehammer, Napoleon commented that it was practically impossible to knock the windmill down. Benjamin, however, knew exactly what the men were about and commented that they were going to place blasting powder into a hole they were making.
Not long after, there was a loud explosion and the animals, except Napoleon, threw themselves flat. After the smoke had cleared they saw that the windmill had been blown away completely. When the animals saw that their desperately hard work had been destroyed, their courage returned and they launched a ferocious attack. They drove the men off the farm even though nearly everyone was injured and some killed in the process.
They had won, but they were weary and bleeding. Slowly they began to limp back towards the farm. The sight of their dead comrades stretched upon the grass moved some of them to tears. And for a little while they halted in sorrowful silence at the place where the windmill had once stood. Yes, it was gone; almost the last trace of their labour was gone! Even the foundations were partially destroyed. And in rebuilding it they could not this time, as before, make use of the fallen stones. This time the stones had vanished too. The force of the explosion had flung them to distances of hundreds of yards. It was as though the windmill had never been.
The animals heard a gun being fired in the distance and Boxer asked what the gun was firing for, Squealer told him that it was to celebrate their victory. Boxer was very upset on hearing this, and for one of the very rare moments in which he disagreed, he responded:
It is obvious that Boxer felt that there was nothing to celebrate since some of his comrades had been killed and practically all of them had been injured in some way or another. To crown it all, their precious windmill had been utterly destroyed. Considering these factors, he had come to a quite logical conclusion - they had not gained anything and had lost much of what they had before.
Squealer explained that they had driven the humans off their sacred soil and Boxer responded that the windmill had been destroyed so that could not possibly have been a victory. Squealer then told him that a windmill could be rebuilt but that they had won back their land to which Boxer responded:
"Then we have won back what we had before,"
Squealer asserted that that had indeed been their victory. Afterwards, when Napoleon made a speech and the flag was flown with the guns firing seven times, the animals felt that they had indeed won a victory.
Boxer's remarks in this instance probably sealed his doom, for, when he fell desperately ill later, he was removed from the farm in a van carrying the name of a horse-slaughterer on its side. Only Benjamin had the ability and awareness to notice this most significant detail and he warned the other animals that Boxer was being taken to his death. The animals tried to stop the van from leaving whilst Boxer tried to break out, all to no avail.
Squealer later explained that the veterinarian had bought the van from a knacker but had not yet removed the insignia. Boxer had indeed, been taken to the hospital where he received only the best care and attention but had, unfortunately, passed away. This explanation pleased the animals and they felt relieved that their erstwhile comrade had died happy.