What is McEwan trying to say about innocence and loss of it and of (relatively) modern society?

The question of innocence is a central idea in this novel.
When Briony decides to lie about what she saw, her perspective is one of a child's
understanding of sex. She can only interpret the act between Cecilia and Robbie as
violent or perverted because of her lack of experience. Despite Briony's relative
innocence as a child, she is capable of manipulating events and ruining peoples' lives.
She tries to atone for this by working as a nurse during the war, and shows depths of
compassion comparing for soldiers she loses her innocence quickly, witnessing the
horrible condition of the wounded men up close. But the lesson comes too
late.