By JON CARAMANICA

For pure star-on-star revenge, “Dear John,” from the new Taylor Swift album, will be tough to beat. Six and a half minutes long and flagrantly provocative, it’s a deeply uncomfortable song, its protagonist anguished and violated.

“Don’t you think I was too young to be messed with?” she asks. “The girl in the dress/ Cried the whole way home.” John Mayer, who tabloids reported Ms. Swift was involved with earlier this year, has brought this out in her, awakening her ire and her creativity.

Rather than write a song in her familiar country-pop mode, she’s written an electric blues, a pointed reminder of Mr. Mayer, who’s a master of the style.

“I feel like in my music I can be a rebel,” Ms. Swift said at a quiet restaurant not far from her new Nashville apartment. “I can say things I wouldn’t say in real life.

I couldn’t put the sentence together the way I could put the song together.” Ms. Swift, while wide-eyed and easily awestruck, is prim and difficult to ruffle. She is still sometimes treated as if she were a child star, but she’ll turn 21 in December. “Dear John” is by far the most scorching track on “Speak Now,” which was released last month, though plenty of the rest of the album stings.

It’s the most savage of her career, and it’s excellent too, possibly her best. In the diminished world of the music business, where album sales have plunged by more than 50 percent in the last decade, genuine blockbusters are an endangered species.

But in the first week of November the recording industry got a rare bit of good news with the unmitigated triumph of “Speak Now.” The album sold 1,047,000 copies in the United States in its first week, making it the fastest- selling new record in five years.

As many in the music industry see it, “Speak Now” proves that Ms. Swift has transcended the limitations of genre and become a pop megastar. Even apparent setbacks - like Kanye West’s outburst at the MTV Video Music Awards last year, when he suggested on stage that another artist should have won an award that went to Ms. Swift ? have ultimately worked to expand her celebrity.

After several days of uncharacteristic silence on her Twitter feed, Ms. Swift wrote to her 4.5 million followers: “I . . .Can’t . . . Believe . . . This . . . You guys have absolutely lit up my world. Thank you.” “Speak Now” touches on many of the major public events in Ms. Swift’s life the last two years - her conflict with Mr. West , the sprouting criticisms of her live singing voice, a relationship with the actor Taylor Lautner, rumored dalliances with Mr. Mayer and others .

“I’ll watch your life in pictures like I used to watch you sleep,” she tells an ex on “Last Kiss.” On “Better Than Revenge,” she seethes over losing someone to a predatory woman. “She’s an actress/But she’s better known for the things that she does on the mattress.” While she’s articulate in song, she admits to struggling in her day-to-day life.

“I can say them at a business meeting,” she said of being direct with words. “But for me, saying them to a person that I really care about in whatever sense, whatever capacity, is a little tougher, because it doesn’t have a first verse, second verse and bridge.

” Ms. Swift still believes in the value of the last word; she’s a far better monologuist than dialoguist. And yet her new adversaries are celebrities who want to tell their own stories, and undoubtedly will. At the 2010 Video Music Awards in September Mr. West was able to steal the narrative back from Ms. Swift. Midway through the show she performed “Innocent,” which is about the incident with him, but he closed the show with “Runaway,” which celebrated his boorishness by poking fun at it. His wit trumped her sobriety .

And that’s probably just the beginning. Certainly something loud can be expected of the outspoken Mr. Mayer, who is probably already working on a riposte to “Dear John.” Asked about the possibility, Ms. Swift appeared concerned. “What do I do now?” she said, her brow furrowing for just a second. “I haven’t thought about this.