What strategy does the author use to express his
own feelings about the war?

I’m pretty sure it’s “By describing the departure of the prisoners, he expresses his own doubts about the war.

Explanation:

in our power to make a world happy - to teach mankind the art of being so - to a character hitherto unknown - and to have, as it were, a new creation intrusted to our hands, are honors that command reflection, and can neither be too highly estimated,