Chris Williams, a Montana medical marijuana grower, faces at least five years in federal prison when he is sentenced on February 1. The penalty seems unduly severe, especially because his business openly supplied marijuana to patients who were allowed to use it under state law.

Yet five years is a cakewalk compared to the sentence Williams originally faced, which would have kept the 38-year-old father behind bars for the rest of his life. The difference is due to an extremely unusual post-conviction agreement that highlights the enormous power prosecutors wield as a result of mandatory minimum sentences so grotesquely unjust that in this case even they had to admit it.

Of more than two dozen Montana medical marijuana providers who were arrested following federal raids in March 2011, Williams is the only one who insisted on his right to a trial. For that he paid a steep price.

Tom Daubert, one of Williams' partners in Montana Cannabis, which had dispensaries in four cities, pleaded guilty to maintaining drug-involved premises and got five years of probation. Another partner, Chris Lindsey, took a similar deal and is expected to receive similar treatment. Both testified against Williams at his trial last September.

Williams' third partner, Richard Flor, pleaded guilty to the same charge but did not testify against anyone. Flor, a sickly 68-year-old suffering from multiple ailments, died four months into a five-year prison term.

For a while it seemed that Williams, who rejected a plea deal because he did not think he had done anything wrong and because he wanted to challenge federal interference with Montana's medical marijuana law, also was destined to die in prison. Since marijuana is prohibited for all purposes under federal law, he was not allowed even to discuss the nature of his business in front of the jury, so his conviction on the four drug charges he faced, two of which carried five-year mandatory minimums, was more or less inevitable.

Stretching Williams' sentence from mindlessly harsh to mind-bogglingly draconian, each of those marijuana counts was tied to a charge of possessing a firearm during a drug trafficking offense, based on guns at the Helena grow operation that Williams supervised and at Flor's home in Miles City, which doubled as a dispensary. Federal law prescribes a five-year mandatory minimum for the first such offense and 25 years for each subsequent offense, with the sentences to run consecutively.

Consequently, when Williams was convicted on all eight counts, he faced a mandatory minimum sentence of 80 years for the gun charges alone, even though he never handled the firearms cited in his indictment, let alone hurt anyone with them. This result, which federal prosecutors easily could have avoided by bringing different charges, was so absurdly disproportionate that U.S. Attorney Michael Cotter offered Williams a deal.

Drop your appeal, Cotter said, and we'll drop enough charges so that you might serve "as little as 10 years." No dice, said Williams, still determined to challenge the Obama administration's assault on medical marijuana providers. But when Cotter came back with a better offer, involving a five-year mandatory minimum, Williams took it, having recognized the toll his legal struggle was taking on his 16-year-old son, a freshman at Montana State University.

"I think everyone in the federal system realizes that these mandatory minimum sentences are unjust," Williams tells me during a call from the Missoula County Detention Facility. But for prosecutors they serve an important function: "They were basically leveraging this really extreme sentence against something that was so light because they wanted to force me into taking a plea deal." Nine out of 10 federal criminal cases end in guilty pleas.

The efficient transformation of defendants into prisoners cannot be the standard by which we assess our criminal justice system. If the possibility of sending someone like Chris Williams to prison for the rest of his life is so obviously unfair, why does the law allow it, let alone mandate it?