But+alas!+his+strength+had+left+him

The power of peasant army is now gone. The people now has no power, but Stalin's the only one with lots of power left because he controls everything. After their loss of Boxer, the other insignificant animals have no choice but to obey Napoleon. If they don't they'll get murdered by those dogs too. Boxer's death represents how much people were easily fooled by Stalin's easy lies, just like how Napoleon has manipulated the stupidity of animals who couldn't read.