Not so long ago, the US believed it had cracked the violent crime epidemic. “Zero tolerance” and “three strikes and you’re out” were the mantras of the 1990s. Declining homicide rates were its product. Today, as the riots intensify in Baltimore following the death of yet another unarmed black man in police custody, such verities no longer hold.

With 2.3m people behind bars, zero tolerance has delivered the largest incarcerated population in the world. US prisons are fuller than those of autocratic China, which has a population more than four times as big. It has also unpicked some of the gains of the 1960s civil rights era. Tens of thousands of black men are in jail for offences that merit little more than a slap on the wrist for their white counterparts. It is little use pointing to the fact that the US has elected its first black president. A generation believes it has been stigmatised and their complaints ignored. As Martin Luther King said before he was assassinated in 1968: “A riot is the language of the unheard.”

Yet amid the gloom, there are signs that US politics is coming to terms with the scale of the challenge. Bill Clinton, who as president was the chief author of the 1990s penal reforms, recently admitted they had “overstepped the mark”. In his day, no politician could afford to be seen as “soft on crime”. Today, both parties accept the need to reduce the US jail population and give those with criminal records a fairer chance at starting over.