Scrambling ever higher through the trees up the goat tracks, Krystyna Skarbek headed towards the Nazi German frontier garrison on the Col-de-Larche, a strategic pass in the Alps. The garrison fort dominated the surrounding terrain, and effectively controlled the military route down to the large French garrison town of Digne.

It was a hot day. Krystyna had tied her hair back with a white and red silk scarf and her…