Who sees the loss in the Battle of the Windmill?

It is Napoleon who is at a loss during the Battle of the Windmill. When Frederick's men attack the farm and blow up the windmill, Napoleon doesn't know what to do. This is meant by Orwell to parallel what happened in the Soviet Union during World War II; indeed, the Battle of the Windmill is an allegory of the USSR's involvement in that conflict. Hitler's invasion of Russia genuinely took Stalin by surprise, even though he'd suppressed all intelligence that indicated an attack was imminent. Stalin didn't believe that Hitler would be foolish enough to launch an invasion while still fighting in Western Europe; a war on two fronts made no sense to him. So in the immediate wake of the Nazi invasion, Stalin was reduced to a state of torpor and indecision.
And it's the same with Napoleon, who of course is an allegorical representation of Stalin in Animal Farm. The animals look set to lose the battle until they start fighting back with everything they've got. They eventually prevail, but it's no thanks to Napoleon's less than inspiring leadership. Nevertheless, the ever-loyal Squealer immediately spins the Battle of the Windmill as a huge personal triumph for Napoleon, the monumental victory that's just been won entirely due to his staggering military genius.