The Obama administration's internal debate over how to respond to the spread of an English-language jihadist Web magazine "spiked after the April 15 Boston Marathon bombing," the Washington Post's Ellen Nakashima reports in a story revealing that the U.S. tampered with online copies of the magazine, known as Inspire. An intelligence official told Nakashima, "You can make it hard for them to distribute it, or you can mess with the content. And you can mess with the content in a way that is obvious or in ways that are not obvious."

The alleged Boston bombing suspects, brothers Dzhokhar and Tamerlan Tsarnaev, had reportedly learned how to build a pressure-cooker bomb from the magazine and had been inspired by extremist sermons in it and other publications on jihadist Web sites.

There are two implicit reasons for the U.S. to sabotage a publication like Inspire. The first is to inhibit the spread of Inspire content that could help people plan attacks, such as bomb schematics. The second and more interesting is to inhibit the spread of the extremist ideology that might make someone want to set off a bomb in the first place. "Unfortunately, I think Inspire magazine is a significant threat to the extent that it disseminates information about how to build a bomb or encourages people to get radicalized," Rep. Adam B. Schiff (D-Calif.), a member of the House Intelligence Committee, told the Post for its story on the government's hacking. "It has shown a dangerous effectiveness."

The first reason is pretty straightforward: make it harder for would-be terrorists possibly like the Tsarnaevs, who have no actual battlefield experience, to learn how to build something like a pressure-cooker bomb. That sort of ultra-basic terrorism technology is probably available to anyone with access to Google and a sense of tenacious curiosity; the directions also appear in the 1971 "Anarchist's Cookbook," which is easily downloaded from well-known music and movie piracy Web sites. But it's true that publications like Inspire do help disseminate bomb plans, putting them into the hands of people who might not have the curiosity or know-how to otherwise find them.

If sabotaging Inspire's distribution could prevent just a few incremental people from getting their hands on bomb plans (or, in a hypothetical scenario it's not hard to imagine U.S. officials considering as well, quietly changing the plans to make them non-functional), then the potential benefit there to the United States seems obvious.

What's especially interesting about this, though, is the second, implicit reason for the United States to hack these sorts of publications: to slow the spread of extremist ideas. As one former official told the Post, "It’s obvious if people are calling for crazies to murder a U.S. citizen, why wouldn’t you stop it?"

But stemming the flow of ideas is not as simple as it might sound, particularly on the Internet. Inspire magazine appears on vibrant jihadist Web forums like Tawhed, where calls for violence against non-believers are frequent and forceful. Those forums are themselves enmeshed into a much larger and more widely dispersed network of jihadist discussion and media, one that reaches across video-sharing sites like YouTube and social networks. Inspire is just one output of a much larger network. If the Tsarnaevs were already accessing issues of Inspire on the jihadist forums and bookmarking terrorism-promoting video sermons on YouTube then it's not clear that blocking them from downloading the Inspire PDF would have altered the ideological course that brought them to the Web magazine in the first place.

By the time people like the Tsarnaevs are familiar enough with the jihadist corner of the Internet to start browsing its Web magazines, it might be too late to change their ideological course by sabotaging Inspire. It's true that the forums themselves can be targeted, and do at times mysteriously go offline. But their users typically just pop up elsewhere. And this tells you something about how hard it is for even skilled, dedicated U.S. cyber teams to slow the spread of ideas and ideology. Hacking can't kill the actual ideas or the people promoting them, it can just encumber their ability to reach one another. But the Internet is just too vast, at once impossibly expansive and intricately compartmentalized across an ever-shifting landscape of forums, media-sharing sites, social networks and chat tools.

Consider the international effort, involving much more than just the United States, to halt the spread of pirated music and movies. This piracy costs the U.S. economy at least hundreds of millions of dollars per year, maybe billions. Given that the files are enormous and typically downloaded by non-tech-savvy people who are willing to only go so far out of their way, the piracy should be easy enough to track and halt. And yet the United States and other countries have seemed largely powerless to stop it. If the U.S. can't stop hungover college kids from exchanging thousands of free copies of "The Avengers" online, can it really hope to prevent a much smaller and more committed contingent of jihadist Web users from exchanging simple ideas, messages or, at the most, video sermons?

The only country that has had any success at slowing the spread of ideas online is China. It required putting all of the people it wishes to monitor within its borders, constructing a giant high-tech firewall around the country, making sure that everyone uses only services approved and indirectly controlled by the government, employing huge numbers of censors who can hand-monitor all online discussion and spending years disseminating official ideology to combat competing ideas. And, actually, it still only works so well, with Chinese Web users getting savvier to the outside world every day and a significant minority of the curious able to freely exchange forbidden information and ideas over virtual private networks that connect them to outside severs.

Is it possible that the United States can make it tougher for jihadists to distribute, say, the plans for how to make a pressure-cooker bomb? Sure. But if the Internet has proven anything over the last 10 or so years, it's that the free flow of ideas and ideology has become largely impossible to stop. That typically works in the U.S.'s favor but this is one case where it doesn't.