A privateer is a private individual or ship authorized by a government during wartime to attack enemy ships and seize their cargo. This authorization is typically granted through a letter of marque, which legitimizes the privateer's actions as a form of warfare. Privateering was common from the 16th to the 19th centuries, allowing nations to augment their naval power without the expense of maintaining a larger navy. However, privateering has largely fallen out of favor and is considered illegal under modern international law.