Why is it ironic that the animals are congratulated at the Battle of the Windmill in Animal Farm?

There is irony in the congratulations of the animals at
the "Battle of the Windmill" for a couple of reasons.  The first is that the windmill is
totally destroyed.  The congratulations of a battle would be in recognition of victory. 
The windmill was not saved, as it was obliterated.  Yet, Squealer understands very
acutely the nature of the animals.  He understands that if all of the animals coalesce
and share anger amongst one another, they can mobilize to get rid of the pigs'
leadership much like all of the animals unified to get rid of Jones and the humans.  In
an attempt to misdirect them and placate them, Squealer declares success in turning back
the humans and creates the "Battle of the Windmill" label and extends the
congratulations.  In doing so, the animals' attention is turned against the humans and
not against the pigs' leadership, whose attempts to try to negotiate with both sets of
human farms ended up causing the destruction in the first place.  It is also ironic that
congratulations was extended because of what follows it.  The pigs' use "The Battle of
the Windmill" as a way to get the animals to rebuild the windmill.  In this light, the
congratulations is empty.  It is a pat on the back, followed by a whipping to get
working on building it again.