What is the main conflict in Among the Brave by Margaret Peterson Haddix?

Trey is the third child of a family. Since families in the story are limited to only two children, third children are illegal and are immediately captured, jailed, or killed by the Population Police, a group organized to enforce the "third child law." The novel's main conflict is centered around Trey's determination to survive, in addition to help Mark, who is the older brother of his friend Lee, rescue Lee who has been captured.