The Alliance Fleet, also called the Rebel Fleet, referred to the various spacecraft operated jointly by the Rebel Alliance in their struggle against the Galactic Empire. Although it was minuscule in size compared to the Imperial Navy, and in many cases its ships were of non-military design, the fleet proved to be a thorn in the Empire’s side throughout the Galactic Civil War.
Following the Battle of Endor and the creation of the New Republic, the Rebel fleet was renamed into the New Republic Defense Fleet.
For the bulk of the Rebel Alliance’s early history, the acquisition of a legitimate Navy seemed impossible. The Rebel Alliance fleet was originally quite small, comprising of small to mid-sized cruisers, converted freighters, and outdated starfighters. Much of the war material had been scavenged from fleets of the former Confederacy of Independent Systems. The Alliance’s main sources for its fleet were defection, theft, or salvage. Genuine construction would take place, but the limited locations made building huge ships difficult.
In terms of organization, the Rebel fleet was scattered and under various command structures. All ships in service of the Rebel Alliance in the early years before the Battle of Yavin were owned and operated by each individual Alliance member, such as the Sullustans, Corellians, or Chandrilans.
These scattered units would work with each other from time to time, such as during the Battle of Ylesia, but overall the space forces of the Alliance were not properly organized and equipped to seriously oppose the Imperial Navy.
As the Galactic Civil War wore on, the ragtag fleet began to grow, as disaffected Imperials switched sides and numerous planets declared their support for the Alliance. When the Mon Calamari chose to join the Rebel Alliance, they brought with them their Star Cruisers, giving the Alliance the firepower needed to contend with Imperial warships.
Mon Mothma realized the need for a single and organized Rebel armada, and decided to appoint the Mon Calamari Gial Ackbar to the rank of Admiral. With the rank came the position of Supreme Commander of the Alliance Fleet.