This video does a good job of breaking down the exponential growth of the prison population and attributes the major part of the growth to the “War on Drugs.”

Also, the Wall Street Journal published a pretty good article about the War on Drugs. One key excerpt:

The total number of persons incarcerated in state and federal prisons in the U.S. has grown from 330,000 in 1980 to about 1.6 million today. Much of the increase in this population is directly due to the war on drugs and the severe punishment for persons convicted of drug trafficking. About 50% of the inmates in federal prisons and 20% of those in state prisons have been convicted of either selling or using drugs. The many minor drug traffickers and drug users who spend time in jail find fewer opportunities for legal employment after they get out of prison, and they develop better skills at criminal activities.