From The Federalist:

Privateering has a rich and storied history. Some 500 years ago, European nations issued letters of marque to supplement their naval forces. The practice allowed private individuals and ships to capture enemy vessels and cargo, effectively outsourcing naval warfare. 

Privateers played a significant role in American history, particularly during the Revolutionary War and the War of 1812, where they disrupted British supply lines and captured valuable prizes. The last time the United States used privateers was during the War of 1812 — though the Confederacy employed the practice in the Civil War.