Metallica are one of the biggest rock bands in the world, but their April 13, 2000, lawsuit against the file-sharing site Napster became a defining moment of their career.

The path to the courtroom began in June 1999, when college dropout Shawn Fanning and teenage hacker Sean Parker launched a program whose moniker came from the former's childhood nickname. Napster's original aim was to create "a way for people to search for files and talk to each other," Newsweek reported in 2000. "'To build communities around different types of music,'" Fanning said in the article.

However, the expansion of high-speed internet, particularly on college campuses, made the program the perfect solution for students eager to search for and download mp3s. Finding rare (or even common) songs became as easy as typing in a band name or a song title in a search bar. Soon, thousands of people were swapping music files like digital baseball cards, using Napster as the conduit.

Naturally, the music industry was not pleased, and acted swiftly. In early December 1999, the Recording Industry Association of America (RIAA) sued Napster. The organization released a statement, which contained this quote from Cary Sherman, senior executive vice president and general counsel for the RIAA: "Napster is about facilitating piracy, and trying to build a business on the backs of artists and copyright owners." Gold Mountain Management's Ron Stone was even blunter in that same statement: "It is the single most insidious Web site I've ever seen."

Metallica wasn't thrilled with Napster either—especially after discovering an incomplete version of "I Disappear" that was receiving radio airplay came from the service. "I got a call from our office the next day: 'It traces back to something called Napster,'" drummer Lars Ulrich recalled in 2013. To add insult to injury, the rest of the band's catalog was freely available as well. Ulrich's reaction? "And we were like, 'Well, they fucked with us, we'll fuck with them.'"

And so in April, the band filed a lawsuit in the U.S. District Court in the Northern District of California against not only Napster, but also the University of Southern California, Yale University and Indiana University.

According to Rolling Stone, the suit charged the entities with "copyright infringements, unlawful use of digital audio interface device and violations of the Racketeering Influenced & Corrupt Organizations Act (RICO)." The remuneration sought? $100,000 per copyright violation. In a press release (portions of which were quoted by Rolling Stone), Ulrich clarified why the band decided to mount a legal challenge. "With each project, we go through a grueling creative process to achieve music that we feel is representative of Metallica at that very moment in our lives. We take our craft — whether it be the music, the lyrics, or the photos and artwork — very seriously, as do most artists. It is therefore sickening to know that our art is being traded like a commodity rather than the art that it is.

"From a business standpoint, this is about piracy — taking something that doesn't belong to you," he continued. "And that is morally and legally wrong. The trading of such information -- whether it's music, videos, photos, or whatever — is, in effect, trafficking in stolen goods."

Napster stood its ground in the face of this challenge, as well as a separate lawsuit filed against them by rapper Dr. Dre several weeks later. It also withstood colleges and universities blocking access to the service (Yale and Indiana University were then dropped from the lawsuit after doing so). However, other colleges such as Harvard, MIT, Stanford and Duke refused to restrict people on campus from using the service. And plenty of musicians and bands came out in favor of the service, including Public Enemy's Chuck D, Limp Bizkit and the Offspring.

The backlash was immediate and severe against Metallica, perhaps because to bolster their case, the band tracked down 335,000-plus usernames of people they alleged downloaded their music illegally and asked Napster to block them. (The company complied.) As a result, the lawsuit started being seen as a personal attack against fans or a greedy move, not a matter of principle or a disagreement between businesses. "Some artists are in it for the pure art of music. Others are in it for the money," student Wayne Chang, who managed Napster's online community bulletin boards, told CNET. "Metallica just showed which side of the line they're on."

The online music company August Nelson set up a website called PayLars.com that MTV reported let fans "'donate' $1 for each officially released Metallica song, to 'make up for all the revenue the band thinks it’s losing to online MP3 trading.'" Aspiring animator/filmmaker Bob Cesca also got in the act, producing several videos (links NSFW) mocking the lawsuit and the band members, and creating an anti-Metallica short called "Metalligreed" for Motley Crue.

“Pigs get fat and hogs get slaughtered, and I think Metallica’s hogs,” bassist Nikki Sixx told MTV at the time. “They make enough off T-shirts and concert events and other forms of corporation. I think that it’s not acceptable behavior for an artist to do that to their fans. Elektra … [and] Metallica’s management, they’re puppeteering the guys in Metallica and they’re fucking their fans, and I think it’s fucked." (To that, Metallica's then-spokesperson, Gayle Fine, replied to MTV: “If Motley Crue’s on one side, and we’re on the other, you can guarantee we’re on the right side.").

Watch 'Metalligreed' (NSFW)

Ultimately, Napster settled their lawsuits with Metallica and Dr. Dre in July 2001. Ulrich for one was satisfied with the outcome, Billboard reported: "I think we've resolved this in a way that works for fans, recording artists, and songwriters alike," he said. "Our beef hasn't been with the concept of sharing music; everyone knows that we've never objected to our fans trading tapes of our live concert performances. The problem we had with Napster was that they never asked us or other artists if we wanted to participate in their business. We believe that this settlement will create the kind of enhanced protection for artists that we've been seeking from Napster."

Napster co-founder Fanning was equally optimistic about the future post-Metallica settlement: "We look forward to gaining Metallica's support and respect as we work to develop Napster into a tool that can be responsive both to artists' needs to communicate their art and the desires of music lovers throughout the world."

Indeed, at the time of the settlement, Napster was in the process of retooling its service. After a long and contentious court battle involving injunctions, stays and attempts to mollify record label requests -- as well as support from acts such as the Dave Matthews Band, who released an official mp3 through the service, and an alliance with one-time adversary Bertelsmann -- Napster shut down the free file-sharing system in July 2001. This stoppage was supposed to be temporary while the company tweaked its copyright-catching filters to be compliant with court orders, and tried to get a new, sanctioned version of its service off the ground. (In fact, terms of the Metallica and Dr. Dre settlements reportedly included a clause where the artists "agreed to allow the sharing of some music once Napster installs a system that ensures payment to artists and publishers.") However, that day never came, and Napster filed for bankruptcy and liquidated its assets in 2002.

When Ulrich looked back on the Napster lawsuit in a 2014 Reddit AMA, he had few regrets. "I wish we had been better prepared for the shit storm that we found ourselves in," he wrote.

"I don't regret taking on Napster, but I do find it odd how big of a part of our legacy it has become to so many people, because to me it's more like a footnote." However, he was still clarifying misconceptions about why Metallica filed the lawsuit, and what they hoped to accomplish.

"I was also stunned that people thought it was about money," Ulrich continued. "People used the word 'greed' all the time, which was so bizarre. The whole thing was about one thing and one thing only - control. Not about the internet, not about money, not about file sharing, not about giving s--- away for free or not, but about whose choice it was. If I wanna give my s--- away for free, I'll give it away for free. That choice was taken away from me."