With a record-setting 2 million people now locked up in American jails and prisons, the United States has overtaken Russia and has a higher percentage of its citizens behind bars than any other country.

Those are the latest milestones resulting from a two-decade imprisonment boom that experts say probably has helped reduce crime but also has created ballooning costs and stark racial inequities.

Overseas, U.S. imprisonment policy is widely seen as a blot on a society that prides itself on valuing liberty and just went to war to overturn Saddam Hussein’s despotic rule in Iraq.

Today the United States imprisons at a far greater rate than developed Western nations and impoverished and authoritarian countries. On a per capita basis, according to the best available figures, the United States has three times more prisoners than Iran, four times more than Poland, five times more than Tanzania and seven times more than Germany.

A major cause of the explosive growth is the war on drugs. In 1980 about 40,000 Americans were locked up for drugs-only offenses. Now the number is 450,000, according to the Sentencing Project in Washington.