America’s biggest mistake over the last half-century arguably had nothing to do with the war in Vietnam or Iraq, or with Watergate or Donald Trump. Rather, I’d say that it was mass incarceration, fueled by the war on drugs.

The United States used to have incarceration rates similar to those of Europe — and then, beginning in about 1970, we increased the number of people behind bars sevenfold. About as many Americans now have a criminal record as have a college degree. Mass incarceration shattered America’s family structure, magnified race gaps, left millions of people marginalized — and has been brutally unfair.

Years ago I wrote about a case that still haunts me. Dicky Joe Jackson was a Texas trucker whose 2-year-old son, Cole, needed a bone-marrow transplant to save his life. The family raised $50,000 through community fund-raisers, but this wasn’t enough — so Jackson tried to earn the remainder by transporting meth in his truck for a distributor. He was caught and sentenced to life in prison.

The prosecutor himself thought the sentence unjust, saying of Jackson: “He didn’t know of any other way to take care of his kid.”

Over the last week, we’ve been focused on Attorney General William Barr’s distortions of the Mueller Report, but many years ago he did something even more damaging. In his first stint as attorney general, Barr in 1992 issued a report called “The Case for More Incarceration.” He was one of many politicians and officeholders, Democrats as well as Republicans, who led the United States, with 5 percent of the world’s population, to hold almost 25 percent of the world’s prisoners.

Finally, America is beginning to unravel this historic mistake.

The best thing the Trump administration has done so far is its backing of the bipartisan First Step Act on criminal justice reform. The act, signed into law by Trump in December, marked a turning point away from mass incarceration, and small numbers of federal offenders have been released early since then.

I saw the new mood on criminal justice while moderating a panel the other day at the Milken Institute Conference in Los Angeles. Beside me was Republican Gov. Phil Bryant of Mississippi, a conservative with whom I agree on nothing else, but he has worked heroically since 2014 to reduce Mississippi’s prison population by 11 percent. This has saved the state $46 million, he said.

Bryant also argued in the panel discussion for ending America’s system of de facto debtor prisons, in which poor people end up jailed because of an inability to pay fines. This is a problem in many states: One day when I visited the Tulsa County jail, 23 people were locked up simply for failure to pay government fines and fees.

Another conservative on the panel, Mark Holden of Koch Industries, spoke eloquently about how our criminal justice system traps people in poverty when they need second chances. He said that the system is so flawed that “it needs to be blown up, quite frankly — in a nonviolent way.”

While the First Step Act was helpful, its practical impact was tiny because the great majority of prisoners are not in federal prisons but are held at the state and local level. Emily Bazelon writes in her excellent new book, “Charged,” about the enormous discretionary power that local prosecutors have over people’s lives, about the misuse of the cash bail system, and about the abuse of plea bargaining. Those are three items for the reform agenda ahead.

One gauge of how far we have to go: Even if we released half of America’s prisoners tomorrow, we would still have several times the incarceration rate of most European countries.

One obvious step is to reverse the decision made 25 years ago to make prisoners ineligible for Pell grants. Schooling reduces recidivism and pays for itself, so it’s just idiotic not to educate offenders.

We also need to broaden the conversation about criminal justice reform. More than half a million American children still suffer from lead poisoning each year, damaging their brains and increasing the risk that they will commit crimes. And if we invested more today in high-quality preschool programs for at-risk youngsters, we would have to worry less in 18 years about criminality.

One sign of progress. Dicky Joe Jackson has been released from prison, after 20 years behind bars.

Another prisoner I wrote about years ago hasn’t been so lucky. Edward Young of Tennessee helped a widow sell her husband’s belongings; among them were seven shotgun shells, and Young put them aside so that children wouldn’t find them. Because Young had committed burglaries many years earlier, as a young man, it was illegal for him to possess any ammunition, even though he had no gun to go with it. So he was arrested and sentenced to 15 years in prison.

There he sits — a reminder of how unjust our justice system has become, and how urgent it is to correct America’s greatest modern mistake.

Nicholas Kristof has been a columnist for The Times since 2001. He has won two Pulitzer Prizes, for his coverage of China and of the genocide in Darfur. You can sign up for his free, twice-weekly email newsletter and follow him on Instagram@NickKristof  Facebook