The introduction of matter/anti-matter power plants into the majority of new Earth Alliance ship designs from 2158 onwards meant that in virtually every confrontation the Romulans were outmatched.
The Romulan fleet had invested heavily into developing artificial quantum singularities as a power source, to try to counter these new power plants. This meant however that they neglected research into the far simpler matter/anti-matter source, and only put limited funds into improving their fusion reactor systems, meaning that no significant improvements occurred in this field until very late in the war.
The U-25A (codenamed ‘Clavicle’), which entered service in 2159, benefited from the new fusion reactor research, and was the first Romulan cruiser that could match most Earth vessels in a confrontation since the advent of matter/anti-matter power in the Earth Alliance ships. The U-25A returned to the flatter shape of pre-war ships, as the Romulan fleet acknowledged that the increase in production time and cost was far outweighed by the improved warp dynamics it offered. The combination of this improved shape and the new power plant gave a top speed in excess of warp 4.2.
The use of Fusion, which relied on ample supplies of deuterium fuel, limited the range, but as with the U-24, the Romulan fleet was now fighting within range of its home bases, so this was no longer a priority. Firepower was still a priority; the U-25A carried an impressive mixed and balanced arsenal. When used cleverly the U-25A was a good as most opposition it would face. In a chase, the weaknesses of the limited range would soon show, but when used to ambush enemy forces the class was devastating. Even in an ambush situation, the U-25A had to destroy its target before reinforcements arrived, as full speed would soon exhaust the fuel supply if pursued.
The other factor about the U-25A that weakened it was the power plant itself. The technical achievement in almost doubling the power output was impressive, but the penalty was that if pushed too hard the reactor could easily overload. Reactor explosion, rather than enemy fire, accounted for half of all U-25A losses. 25 U-25A were built in total, of which 15 saw Romulan war service, and of these 11 were destroyed. The limited numbers produced, and the lateness of the introduction into service meant that the U-25 had only limited influence on the outcome of the war. It did, however, reassert the Romulan style of ship design that later became synonymous with all their vessels.
Post-war improved U-25B and C variants saw service, some of these ships survived in the fleet until 2200. The U-25B (2165) introduced improved shields (a major lesson learnt from the war) and an improved plasma cannon. The final version, U-25C changed the balance of the design by mounting more, less powerful starbombs, to compensate for the fitting of a second plasma cannon, as well as enhanced beam weapons.