In the spring of 2005, Tarek Mehanna began translating radical Arabic books and videos into English for the website At Tibyan. The materials had an undeniable flavor of terrorism, encouraging readers to join al-Qaida and kill American soldiers in Iraq. But even the government acknowledges that Mehanna never translated anything at the direct behest of al-Qaida operatives—which is why it’s rather surprising that the government successfully prosecuted him for providing “material support” to a terrorist organization and sentenced him to 210 months in prison.

Everybody agrees that Mehanna supported al-Qaida’s cause; At Tibyan is a fairly popular terrorist forum, and Mehanna translated its content with the clear intention of swaying opinion toward the jihadist cause. But translating, publishing, and praising ideological texts, no matter how morally vile, is generally considered to be a basic free speech activity. Everyone knows that the First Amendment protects translations of Mein Kampf. Why did Mehanna’s translation of jihadist hosannas land him behind bars?

That’s the question Mehanna is asking the Supreme Court, which will decide whether to take his case later in September. If the justices do agree to review Mehanna’s conviction, it’ll be wading into a constitutional controversy at once timeless and novel. The court has long recognized that cheerleading for terrorism may eventually cross the line from free speech to a criminal act. But the Internet’s ability to spread ideas and connect like-minded people may now force the justices to reconsider that boundary. And if the court lets Mehanna’s conviction stand, it may wind up drawing the line dangerously close to the kind of Internet activity some of us engage in without a second thought.

The Supreme Court has been grappling with the question of dangerous speech for as long as America has been attempting to suppress dissent in the name of national security—that is, pretty much forever. For our first century or so, nobody thought that criminalizing anti-American speech raised a real First Amendment concern. But in a 1919 case called Abrams v. United States, Justice Oliver Wendell Holmes, joined by Justice Louis Brandeis, cracked the orthodoxy wide open, issuing a stunning dissent from the conviction of a revolutionary anarchist who advocated a violent overthrow of the U.S. government. Holmes described a constitutional command to tolerate anti-government speech, explaining that censorship of “opinions that we loathe and believe to be fraught with death” is unacceptable unless those opinions “imminently threaten” the country’s safety.

Today, Holmes’ view is more or less the law. But that hasn’t kept Congress from passing censorship laws on the theory that a certain group’s speech qualifies as a clear and present danger to the country. The latest attempt, a post-9/11 anti-terrorism law, made it illegal for anyone to “knowingly” provide “material support or resources” to a foreign terrorist organization, even for nonviolent purposes. What if that material support comes in the form of otherwise protected speech? That’s OK, said the Supreme Court in a 2010 called Holder v. Humanitarian Law Project—so long as the terrorist sympathizer truly “coordinated” with a terrorist organization.

This “coordination” standard appears nowhere in the law itself; the court wrote it in, since, without it, the statute would be a hopelessly unconstitutional suppression of speech. But where, exactly, does free speech end and unlawful terrorist coordination begin? We’ll save that question for another day, the court wrote, when a “concrete fact situation” presents itself.

Mehanna is hoping the court will view his prosecution as precisely the kind of “concrete fact situation” it prognosticated in 2010. But even if the justices vote to take his case, it’s no sure thing that they’ll grant him First Amendment protection. There’s little doubt that Mehanna wanted to help terrorists; it seems the only thing that stopped him was his own utter fecklessness. He flew to the Middle East to join a terrorist training camp but couldn’t find any; he was intent on killing American troops stationed in Iraq but got cold feet once he arrived in Fallujah. When an actual al-Qaida associate finally got in touch in Mehanna asking him to translate a video, Mehanna seems to have overlooked the email; when the associate followed up six months later, Mehanna was puzzled, responding, “what do u mean.”

The government’s case, then, is built on the tenuous theory that by associating with al-Qaida associates and promoting jihadist speech, Mehanna provided coordinated material support to terrorists. There is, however, a problem with this theory: It’s exactly what the government promised it wouldn’t do in Humanitarian Law Project. During oral arguments for that case, then-Solicitor General Elena Kagan told Justices Ruth Bader Ginsburg and Anthony Kennedy that Americans could “absolutely” still meet and communicate with terrorists, and advocate for their causes. The one activity the law forbade, Kagan agreed, was “communicat[ing] advice” to terrorists “on how to pursue their goals.”

Why is this distinction between “advocacy” and “communication” important? Because without it, we may accidentally become a nation of criminal terrorist sympathizers. The Internet fosters association and amplifies advocacy. Under the government’s revised interpretation of the law, an American who uses the Web to defend terrorist groups might find herself facing federal prosecution.

This scenario might seem far-fetched—but in reality, it’s chillingly plausible. Imagine an American driven to sympathize with Hamas, a designated foreign terrorist organization, after seeing images of brutality in Gaza. She posts Facebook statuses supporting Hamas’ mission to “free Gaza” and converses with other Hamas supporters—including a few associates of the organization—on a blog. Eventually, she begins translating Hamas’ statements and posting them online at the behest of her newfound comrades. All of this activity would seem to be a constitutionally protected form of speech and association. But under the government current theory, our Hamas sympathizer is, in fact, a criminal who has “coordinated” with terrorists to provide them with “material support.”

Mehanna hopes tech-flavored hypotheticals like these will convince the Supreme Court that the law (as the government construes it) is an infringement on his free speech rights. He’s also arguing that this interpretation of the law in unconstitutionally vague—that is, its contours are so blurry that Mehanna couldn’t have known his actions were criminal. It’ll be a close call. The justices have a mixed record in applying legal principles to new technology; sometimes, they do a decent job; other times, they get confused about GPS trackers and tiny constables.

But for a dispute that’s fundamentally about speech on the Internet, Mehanna’s case isn’t really that much different from the one the court grappled with back in 1919. Posting jihadist translations on a Web forum is essentially a modern version of handing out violently anarchist pamphlets in the streets. In 1919, a few hundred people might read your calls to action; today, a few billion could. That prospect might sound scary—but scariness isn’t usually a reason to send a man to prison for his advocacy. With Mehanna, the government has functionally admitted that it doesn’t think terrorist sympathizers deserve free speech rights. It’s the same old drama we’ve seen for a century, with jihadists playing the familiar role once occupied by anarchists and communists. The bogeyman, and the technology, may change. But our fears remain largely the same.

This article is part of Future Tense, a collaboration among Arizona State University, the New America Foundation, and Slate. Future Tense explores the ways emerging technologies affect society, policy, and culture. To read more, visit the Future Tense blog and the Future Tense home page. You can also follow us on Twitter.