As originally written, the Constitution of the United States not only countenanced slavery, it protected it. On January 1, 1863, President Abraham Lincoln issued the Emancipation Proclamation abolishing slavery in areas of the United States engaged in rebellion against the government, but this was a war measure, subject to reversal by any subsequent statute or court ruling. Lincoln led the fight for the enactment of the 13th Amendment which provides:
But slavery has not disappeared from the Earth - it persists today, even within the United States.
On June 16, 2009, in issuing the annual Trafficking in Persons Report, Secretary of State Hillary Clinton made these remarks:
Here are three studies and reports describing the extent of these practices internationally and in the United States.
U.S. State Department Trafficking in Persons Report 2009. The linked page contains links to the various portions of the report. I recommend first visiting the maps to learn which countries have the highest incidence and most serious forms of slavery and human trafficking. The report also contains a section describing U.S. Government Domestic Anti-Trafficking Efforts, as well as a list of nine Relevant International Conventions prohibiting various forms of human trafficking.
June 2009 Attorney General's Annual Report to Congress and Assessment of U.S. Government Activities to Combat Trafficking in Persons Fiscal Year 2008, making eight general and several specific recommendations for improving the United States' response to human trafficking, mainly regarding improved coordination among public agencies and increased awareness of the problem within law enforcement and the larger community.
January 2009 Bureau of Justice Statistics Special Report, Characteristics of Suspected Human Trafficking Incidents, 2007-2008, showing that authorities investigated over 1200 incidents of human trafficking, and confirmed the incident in over 100 cases in the United States.
The federal government has enacted a number of laws against slavery and human trafficking, including the following:
Peonage, Slavery, and Trafficking in Persons, 18 U.S.C. 1581 et seq., making slavery, the slave trade, and related activities a crime.
Trafficking Victims Protection Act (TVPA), 22 U.S.C. 7101 et seq. The introductory section to this law states:
Sex Trafficking of Children by Force, Fraud, or Coercion, 18 U.S.C. 1591, making sex trafficking of children a crime.
Prevention of Domestic Trafficking in Persons, 42 U.S.C. 14044, requiring the Attorney General to hold an annual conference and issue an annual report on human trafficking.
And here is a link to the National Human Trafficking Resource Center, a national, toll-free hotline for reporting incidents of slavery and human trafficking. The number is 1-888-3737-888.