in certain circumstances to seize property at sea.
Letters of marque and reprisal were once common tools for countries with small naval forces to augment their militaries by drawing upon the strength of their private merchant vessels. As a young country, the United States used the instruments with success in several early conflicts, including the Revolutionary War and the War of 1812. Over the course of the 19th century, however, commissioning private parties to use armed force fell out of favor in domestic and international practice, and Congress has not authorized a President to issue the instruments since the Civil War.
.... Governments increasingly commissioned privateers to help achieve national military and foreign policy goals rather than to vindicate private wrongs. Countries that did not maintain large navies, including the 18th and 19th century United States, often relied on privateering as a cost-effective way to supplement their naval forces. Letters of marque and reprisal thus enabled governments to invoke the resources of private maritime enterprise to achieve state military objectives. Privateers typically aimed to disrupt an enemy's commerce rather than engage with and destroy its military vessels, making the instruments tools of economic statecraft.
Recipients of letters of marque and reprisal obtained government authorization to engage in hostilities, but they did not receive government funding to embark on their endeavors, and they were required to outfit vessels at their own expense. Privateers were incentivized to aid the government because they received a large portion of the proceeds of the sale of captured vessels and their cargo as prizes of war. Privateers might also receive direct payments, called bounties, for some accomplishments, such as sinking superior armed vessels or capturing prisoners. Because of these financial awards, privateering became a lucrative industry during some periods, including the American Revolution.
https://www.congress.gov/crs-product/LSB11272
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