The violent conflict of the alliance of United Earth (UE) and the Andorian Hegemony against the Romulan Star Empire, known today as the Earth-Romulan War (2156-2160), arose from an antagonistic first contact between the sides and a subsequent invasion of known space by Romulan forces. Until the conflicts with the Borg and the Dominion in the late 24th century, the war was the most destructive interstellar conflict involving Federation members.

Before hostilities started, the UE had enjoyed more than 50 years of economic growth and peaceful expansion and had established colonies on worlds up to 6 pc away from Sol. Humans had established friendly relations with the Vulcans, Andorians, and Tellarites. The only known threat was from Orion raiders, who had launched sporadic attacks against UE and Andorian colonies in the Libra Sector since the 2120s. When unidentified alien scout ships were detected near the Utopia, Mohannak, and Tlolu colonies starting in 2152, the United Earth Stellar Navy (UESN) responded with intensive surveys of neighboring systems to establish contact with the unknown race. Few people anticipated that these events would lead in March 2156 to a Romulan attack on UESN Stations Shen-shu and Yu-lei in the First Battle of Hell's Gate, which launched 4 years of war. After withstanding a furious assault against the UE's outer colonies, the UESN turned the tide in October 2157 by destroying a Romulan invasion fleet in the Battle of the Vela Gap. The UESN then launched a counterattack against Romulan-held systems, advanced deep into enemy territory, and forced the Romulans to sue for peace with the decisive Battle of Cheron in August 2160.

Earth-Romulan War, 2156-2160

The effects of the war continue to be felt to this day. The victorious alliance became the basis of the United Federation of Planets, which was founded in 2161. The UESN became the nucleus of Starfleet, the Federation's exploratory and defensive organization. New technologies developed during the war allowed the confident young Federation to rapidly expand its influence across this part of the galaxy. After their defeat, the Romulans cut off all contact with the Federation and would remain in isolation for the coming century.

This Starfleet Museum history focuses on the starships in service with the UESN before and during the Earth-Romulan War. By today's standards, these ships were extremely primitive: many warships were massive, 500,000-ton dreadnoughts that required crews of over 1,000, took months to cross the average 5-ly gap between star systems, and were armed with laser cannons and warp-capable nuclear missiles. All ships were powered by deuterium fusion reactors until matter/antimatter reactors were introduced in 2157. Interstellar ship-to-ship communication was slow, with subspace transmitters and active sensors carried by only specially designed ships. Despite their limitations, the ships of the UESN were instrumental in defending Earth's colonies and defeating an implacable alien foe. These ships paved the way for a new era of interstellar peace and heralded a renaissance in starship design in the Federation.