Ask just about anyone about the cause of mass incarceration in America, and they will point to the war on drugs. It is a conventional wisdom recited by the president, major political candidates, and news reports.

But it's not true.

The following chart from the Prison Policy Initiative breaks down the 2.3 million people in a US jail and prison, based on the inmate's most serious offense (click here for a bigger version):

Drug offenses are, to be sure, a significant contributor, making up about 21 percent of the jail and prison population. But the most common offenses are violent crimes, which nearly 40 percent of the prison population is in for.

The distinction matters: To shed America's status as the world's leader in incarceration, some criminal justice reformers say the country's incarcerated population should be cut by 50 percent. But even if the US released its entire population of drug offenders, it wouldn't be able to achieve that.

Yet much of the talk about reform has focused on nonviolent drug offenses. There are of course good arguments that people shouldn't be arrested or locked up — especially for as long they are — for drug crimes, but it's simply not the case that these people make up a majority of the incarcerated population.

There are some caveats. About half of the federal prison system, which makes up a small but significant chunk of the incarcerated population, is drug offenders, so any reform focused on drug offenses at the federal level — as the Senate bill focuses on — could make a substantial dent in the federal prison population. And the war on drugs is likely creating some violence in the US, since the black market for drugs creates a huge source of revenue for criminal organizations to fight over, and such violence may cause some violent crime.

But if the US wants to end mass incarceration, the chart above makes it clear that the country is going to have to look beyond nonviolent drug offenses.

Watch: The racism of the US criminal justice system, in 10 charts