The Darkest Hour by Caroline Tung Richmond

Lucie is a covert op during World War II, stationed in France because she grew up speaking French. Her job is to take out spies and to uncover plots against the Allies. When she and her fellow ops find out about a terrible plot that could endanger the entire world, Lucie believes she is doing the right thing. But only time will tell if what she thinks is true, actually is. 

Recommended for mature 8th graders and up due to content.