Following the success of Bend It Like Beckham, Knightley landed the role of Elizabeth Swann, in the 2003 American fantasy swashbuckler film Pirates of the Caribbean: The Curse of the Black Pearl.
The film, which was based on the Pirates of the Caribbean attraction at Disney theme parks, saw Knightley, Orlando Bloom, and Johnny Depp play the roles of 18th century pirates.
Knightley was cast in the role because of her close resemblance to Portman; even the two actresses' mothers had difficulty telling their daughters apart when they were in full makeup.
In her first major role, the 2001 Walt Disney Productions feature film Princess of Thieves, Knightley played the daughter of Robin Hood.
To prepare for the film, she trained for several weeks in archery, fencing and horse riding.
During this time, Knightley also appeared in The Hole, a thriller that received a direct-to-video release in the United States.
The film's director Nick Hamm described her as "a young version of Julie Christie".
The two sequels were conceived in 2004, with the writers Ted Elliott and Terry Rossio developing a story arc that would span both films.
Co-starring Molly Parker and Harry Eden, the film had its world premiere at the 2002 Toronto International Film Festival.
To prepare for their roles, they underwent three months of extensive football training under the English football coach Simon Clifford.
Knightley became known for starring in period dramas such as Pride & Prejudice (2005), for which she earned nominations for the Academy Award for Best Actress and the Golden Globe Award for Best Actress; Atonement (2007), for which she was nominated again for a Golden Globe; Silk (2007); The Duchess (2008); A Dangerous Method (2011); and Anna Karenina (2012).
She has also appeared in a variety of other genres, including the romantic comedy Love Actually (2003), as Guinevere in the historical action King Arthur (2004), the psychological thriller The Jacket (2005), biographical action Domino (2005), the historical romantic drama The Edge of Love (2008), the film noir London Boulevard (2010), the dystopian science fiction Never Let Me Go (2010), the romantic drama Last Night (2010), and the dark comedy Seeking a Friend for the End of the World (2012).
Knightley, who was initially skeptical of the project, went on to win the London Film Critics' Circle for Best Newcomer and the Online Film Critics Society Award for Best Breakthrough Performance.