LATE last year, a jury in Boston convicted Tarek Mehanna, a 29-year-old pharmacist born in Pittsburgh, of material support for terrorism, conspiring to provide material support to terrorists and conspiring to kill in a foreign country, after a 35-day trial in which I testified as an expert witness for the defense.

On April 12, Mr. Mehanna was sentenced to 17 and a half years in prison. Hearing this, most Americans would probably assume that the F.B.I. caught a major homegrown terrorist and that 17 and a half years is reasonable punishment for someone plotting to engage in terrorism. The details, however, reveal this to be one of the most important free speech cases we have seen since Brandenburg v. Ohio in 1969.

As a political scientist specializing in Islamic law and war, I frequently read, store, share and translate texts and videos by jihadi groups. As a political philosopher, I debate the ethics of killing. As a citizen, I express views, thoughts and emotions about killing to other citizens. As a human being, I sometimes feel joy (I am ashamed to admit) at the suffering of some humans and anger at the suffering of others.

At Mr. Mehanna’s trial, I saw how those same actions can constitute federal crimes.

Because Mr. Mehanna’s conviction was based largely on things he said, wrote and translated. Yet that speech was not prosecuted according to the Brandenburg standard of incitement to “imminent lawless action” but according to the much more troubling standard of having the intent to support a foreign terrorist organization.