The Empire has fallen and a New Republic is born out of the conflict. But in the ashes of civil war, yet another life stirs...
The Assault Frigates used by the Rebellion and the New Republic which followed were powerful starships that posed a serious threat to anything of a similar class. They weren't Star Destroyers by any means, but they needed little support to defend against most threats smaller than the aforementioned Imperial flagships.
Specifically, the Mk1 variant of the Assault Frigate was actually not an entirely new ship on its own but rather, was an extensive modification of the Republic-era Dreadnoughts that were still in use by the Empire and later the Imperial Remnant.
Engineers stripped the original Dreadnought ships of a significant amount of armor and components, leaving the core of much of the starship exposed but allowing for a skeleton crew to operate the vessels. Addition power supplies were added that offered greater offensive capability and the engine arrangement was modified in order to accommodate the new powerplant. Additionally, several of the smaller laser cannons were removed in favor of two addition medium lasser canons, mounted near the rear of the ship.