Letter of marque

Tertiary by nature, well-sourced for the institutional outline. The substantive history is drawn from the standard maritime-law and international-law literature; the authoritative monographs to chase if a specific claim matters are Janice Thomson's Mercenaries, Pirates, and Sovereigns (1994) and the older Stark The Abolition of Privateering and the Declaration of Paris (1897). For the constitutional-history side specifically, the CRS sidebar is the better citation than this Wikipedia entry.

For this corpus the Wikipedia article is the appropriate citation when the argument needs only the broad institutional outline (what a letter of marque was, when it was used, why the practice ended); for any closer historical claim, follow the article's own footnotes outward.