A "gunboat" is a small combat vessel, carrying one or more guns, that is large enough to sail in the ocean but has a draft light enough to sail in rivers as well. Throughout naval history, gunboats have been valued for their maneuverability, versatility, cheap cost, and rapid construction time.
The original incarnation of the gunboat arose in the 18th century as a small, undecked sailing vessel typically armed with a single forward-mounted cannon. These gunboats would use their superior maneuverability to bring their single gun to bear on larger, slower enemies, and would work in concert with other gunboats and mid-sized ships to overcome superior vessels. A good example of this type of gunboat was the 53-foot Philadelphia, which served as Benedict Arnold's flagship during the Battle of Valcour Island on Lake Champlain in October of 1776. Even though Arnold had larger vessels under his command, he chose a gunboat to carry his flag due to its ability to quickly maneuver him around the battle site.
Gunboats continued to evolve over the course of the 19th century. In the American Civil War, steam-powered, armor plated gunboats battled it out for control of the Mississippi River. In the latter half of the 19th century, European colonial powers frequently made use of gunboats to enforce their will on weaker nations, most notably China, where they inspired the term "gunboat diplomacy." In an age before airplanes, the gunboat's ability to sail up rivers was often the only way to get guns and troops into the interior of a hostile nation.
In the 20th century, gunboats played a prominent role in the Vietnam War where they proved one of the only ways for American troops to patrol vast swaths of dense jungle.