Death for a Cause
"I had it all wrong,' he [Evan] says. 'Before i found you, I thought the only way to hold on was to find something to live for. It isn't. To hold on, you have to find something you're willing to die for.'" (281)
This sentiment explains why humanity is fighting the Others even after four waves of devastation have swept away their loved ones. Living is no longer possible or pleasant in the face of the Others, so the least humans can do is die for each other and those who came before them. All of the characters in the book have found something to die for: Cassie for Sammy, Ben for Sammy/Nugget/squad, Evan for Cassie. It is what keeps them going in the face of such hardship.
Hope as a Secret Weapon
"And hope pulling them out didn't deactivate them... What?' I ask. 'Too much hope?' The side of her mouth twitches. 'Maybe that's our secret weapon.'" (315)
Ringer tells Ben that hope is now their secret weapon when they realize that they can possibly outsmart the Others. They have finally learned the Others' plan, which is to use child soldiers as the fifth wave, with implants in their necks to kill them if they don't comply.
Run = Die, Stay = Die
"He knows what you know: Run = die. Stay = die." (44)
"Retreating is not an option. Advancing is not an option. Staying put is not an option. There are no options. Run = die. Stay = die." (291)
This theme runs throughout the course of the book. It is the theme of all humans who have survived thus far. They are caught between a rock and a hard place. Often, if one thing doesn't kill them, then another probably will. Both Cassie and Ben, quoted above respectively, say this theme to themselves. It is despondent but also acts to stir them to action.
Loss is Universal
"I don't know what your chain is,' she [Ringer] goes on, warm breath in my ear. 'It's different for everyone. They know. Wonderland tells them. It's the thing that made them put a gun in your hand and it's the same thing that chains you to the target.'" (251)
Ringer tells Ben that everyone has something or someone that they have lost when she sees the silver locket. Ben carries the locket as symbol of his sister Sissy, and Cassie carries Sammy's teddy bear. The prevalence of this is so great that one of the Others, Commander Vosch, even goes so far as to fabricate his own. When Ben first arrives at the base, he shows him a picture of Vosch with a wife and child beside him, implying that these are people he has lost to the Others, and thus gaining Ben's trust and sympathy. It is likely that Vosch learned this trick from studying people through Wonderland.
After You've Killed the Weak, the Strong Remain
This is repeated as a reason that the Others should have killed humanity quicker, or outright. Now the people who weren't killed before are the strongest of the bunch. They were always the ones most likely to survive and they are the ones fighting back, with the most chance of succeeding at that too.
You're Safe Now
"The soldiers are heavily armed, but they smile and their voices are gentle. It's okay. Time to get up. You're perfectly safe now." (194)
This is repeated throughout the first chapter narrated by Sammy, The Winnowing. This theme shows the duplicitous nature of those in charge of the camp, even before we find out that they are the Others. They continually push this on the children, saying they are safe now, until Sammy begins to suspect that something isn't right.
Attractiveness Makes for Trustworthiness?
"'You'd take it in a heartbeat if you thought I was hot.' I wondered if he was right. If Ben Parish had dug the necklace out of the pit, would I have taken the gift?'" (70)
"The thing that gets me is whether I'd take that hand if it belonged to anyone other than Ben Parish." (430)
Cassie struggles with the implications of her attraction to Ben Parish, debating whether she trusts him per se or his attractiveness.
