One of Samantha Power's favourite lunch spots is a place off Harvard Square in Cambridge, Massachusetts called Casablanca. Decorated with 20ft murals of the movie, Bogart and Bergman gaze with melancholy at diners digging into their seared cod and mixed greens.

The theme has echoes of Hitler and of Hollywood, which resonate because Power's seminal writings on war and human rights have made her a celebrity favoured by the American left.

Heads turn as she strides past Bogey and Bergman and slides into a banquette. Power seems not to notice. She is so focused that I'm a little surprised she has not come dressed like a distracted professor (she lectures in public policy at Harvard's John F. Kennedy School of Government). She wears a stylish leather coat, black slacks and a starched, striped muslin shirt with a silver and turquoise necklace. Long and lean, she has intense blue eyes and voluminous auburn hair. With a fedora she might look a little like Bergman, but with freckles.

She is equally distinguished in accomplishment. Over-achievement is de rigueur in Cambridge, Massachusetts, but rarely does it come so globally at the age of 33. In her best-selling book of 2002, A Problem from Hell: America and the Age of Genocide, Power chronicled the role of the US in the history of genocide. The book criticises America's record of passivity in the face of international slaughter and has become required reading for anyone hoping to strengthen US foreign policy on human rights. Power pushes the issue as founding executive director of the Carr Centre for Human Rights at Harvard, where her obsessive tendencies have not gone unnoticed. (When she was working on the book she would crank the heat up to 80 deg F during the day so she could stay warm while she worked late into the night....