A pivotal role as Lysander in Michael Hoffman's star studded adaptation "William Shakespeare's A Midsummer Night's Dream" (1999) increased the actor's visibility to an American audience. Virtually naked for much of the film and given the unenviable task of nude bicycling, West still capably held his own alongside co-stars Christian Bale, Calista Flockhart and Anna Friel with a notably charming turn. After this co-starring role, he could be seen on the big screen with an uncharacteristic bit part, uttering a single line as a mostly obscured palace guard in the summer blockbuster "Star Wars: Episode I - The Phantom Menace.” While the role was really very small, West considered the opportunity to be in such a monumental film (an opportunity afforded him as he and star Ewan McGregor shared the same agent) as one not to be passed up. He marked his US television debut featured to a greater extent as Scrooge's nephew Fred in TNT's 1999 presentation of "A Christmas Carol.”

The actor's profile continued to rise through 2000, beginning with his co-starring role alongside Sandra Bullock in the comedy/drama "28 Days,” chronicling New York City writer Gwen's experience in court-ordered rehab. West played Jasper, Gwen's fun-loving British boyfriend who shared her life of hard partying and forgotten evenings. His portrait of the sincere but irresponsible man was engaging and hard to resist, if somewhat two-dimensional. West followed up playing a rhythm guitarist for popular hard rock band Steel Dragon with sardonic glee in the fact-based comedy "Rock Star" (2001), starring Mark Wahlberg and Jennifer Aniston, then got a major career boost when he played Fred Casely, the victim in the ballyhooed murder trial of Roxie Hart (Renee Zellweger) in director Rob Marshall's acclaimed 2002 film version of the tried-and-true musical "Chicago." The actor then appeared in "Mona Lisa Smile" (2003) as a predatory Italian language professor at an all-girls school who casually sleeps with his students. Next, West essayed a man who is told his child never existed, and embarks on a harrowing investigation alongside similarly bereft parent Julianne Moore in the paranormal thriller "The Forgotten" (2004).

In his first turn on the small screen, West did his finest work to date, ditching the Queen’s English for a tough Baltimore twang on David Simon’s gritty crime drama "The Wire" (HBO, 2002- ). West played homicide detective Jimmy McNulty, a hard-drinking outsider who enjoys the hell out of bucking the system, damn the consequences. He joins a joint homicide and narcotics team to take down a notorious drug kingpin (Wood Harris), but discovers that trying to make a difference can ruin one’s career. Subsequent seasons of “The Wire” saw McNulty demoted to the Marine Unit during an investigation of dock workers stealing shipping containers, then retuned to walking a beat in uniform while helping to keep four high school students stay on the straight and narrow. Returning to features, West portrayed Theron of Acragas, tyrant of Greek-occupied Sicily, in “300” (2007), a loose telling of the famed Battle of Thermopylae where 300 Spartan warriors led by King Leonidas (Gerard Butler) inflicted heavy damage to the massive Persian army of Xerxes I (Rodrigo Santoro).

In addition to his flourishing film and television career, West had to his credit London stage performances, starring in productions of "Cloud Nine" and "The Seagull" during director Peter Hall's 1997 season at the Old Vic. He was also featured at the famed Almeida in productions of "The Silver Tassie" and "The Plough and the Stars,” both by Sean O'Casey. In spring 2001, West joined fellow Brits Jennifer Ehle and Alan Cumming in the Broadway revival of "Design for Living.”