Animorphs.
Cool friends fighting aliens...more accurately, the pariah of a fascist civilization, minutes before being eaten alive in front of said "ccol friends" persuades and then bioengineers human child soldiers to facilitate an end to an ill-conceived and failing war now reduced to unilaterally exterminating a parasitic, physically disabled species, itself undergoing a violent civil rights movement on their own planet based on their self-recognized flaws, struggling to realize its place in a universe where godlike beings exist and decide not to offer remedy(rules of the god game) and spectate while the parasites overwhelm all vulnerable species in the known universe.
The child soldiers agree to resist the parasites, but at least a minority of them believe genocide is the wrong answer. After being physically and emotionally tortured, shot, repeatedly disemboweled and having their limbs hacked or bitten off by hosts of the parasitic species, however, all of the child soldiers begin taking violent, morally devastating actions that end their lives as they know them.
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Revelation, war, and loss.
Crossing more lines they hadn't crossed before.
Title
I think it's interesting that visser 1 uses the morph from the first book, signaling that the story we started with us being it's conclusion.
Humans start to die, the war is extending beyond the animorphs, it's too big for just them and they can no longer protect everyone by going on there missions they choose.
They have to aid those dedicated to hopeless caused.
