A number of Nations used privateers to prey on their adversary's vessels. A private vessel might be issued a letter of marque that allowed the privateer to attack ships of a specific nation (or nations), without being accused of piracy.

During the wars between Alliance of Free Stars and Republic of Cinnabar, both belligerents used privateers. In 5147 CE, they constituted a threat to Cinnabar shipping, because the RCN was too tightly stretched to patrol against them.1:1

Several decades before 5147 CE, when Admiral Torgis was presumably a junior officer, he rescued the son of a Cassis prince from an Alliance privateer, earning him a gift of a fine scaleware service.2:14

Cinnabar employed privateers as well, and Adele Mundy considered it very plausible to use them in her cover story.5119 CE

Privateers were not the most reliable forces, and were often barely removed from pirates. Some operated with warrants or letters of marque from multiple governments, and determined who to attack based on their own self-interest rather than any loyalty.6:12

The Estremadura was a privateer, and gave a lot of trouble to the Princess Cecile and Daniel Leary.9:16

A privateer is a private person or ship authorized by a government by letters of marque to attack foreign shipping during wartime. Privateering was a way of mobilizing armed ships and sailors without having to spend public money or commit naval officers. They were of great benefit to a smaller naval power or one facing an enemy dependent on trade: they disrupted commerce and pressured the enemy to deploy warships to protect merchant trade against commerce raiders. The cost was borne by investors hoping to profit from prize money earned from captured cargo and vessels.