There is evidence from the first series ("Court Martial", "The Mark of Gideon", "Whom Gods Destroy", etc.) that the Federation experienced a terrible and long war in the 2250's. This war ended in the Treaty of Axanar.

From the Dominion War, we saw the Federation using whatever ships were available. Though we didn't see the entire war, we did see that Starfleet used for one fleet-the Ninth Fleet-aging Mirandas and Excelsiors.

Going back a century earlier, the Federation is short on ships for this war and needs ships for various missions. An available class is a civilian class of freighters and transports. These ships, which were introduced before the 2240's, are seen by Starfleet to be ideal for cargo and transportation. A few of the ships are converted to Starfleet specifications, given new names and registries, and assigned to missions. One of these ships is the U.S.S. Antares NCC-501. She is the first of the converted civilian ships, giving her name to a class of ships that would come to include the S.S. Norkova, S.S. Xhosa, and U.S.S. Hermes NCC-10376.
After the war, the Antares Class ships are used for ferrying cargo and passengers and scouting missions.

The S.S. Norkova, built in the 24th century, and the S.S. Xhosa, built in the 23rd century, are ships that served in the Starfleet and were later sold to civilians. The U.S.S. Hermes NCC-10376, which is used in 2368 and built in the early 24th century, is probably the last of the Antares Class ships to be used by Starfleet. (Capt. Picard led a fleet that had orders not to engage the Romulans in combat. Instead, the mission is to blockade the Romulans. When the tachyon web is disrupted, Capt. Picard orders a retreat. This is why probably the U.S.S. Hermes NCC-10376 could be involved in this mission. Her function is to help with the creation of the tachyon web and not to engage the Romulans in combat.)

The S.S. Vico NAR-18834 is an example of Starfleet selling ships to civilians when the ship can no longer serve the needs of the fleet. She still carries the markings that she had when she served Starfleet. Interestingly, the types of ships that Starfleet seems willing to sell to civilians are freighters, transports, scouts, and shuttlecrafts. The more warlike ships, such as the Miranda Class, are put into the mothball fleet and are never sold.

By the mid 24th century, the class designation Antares is used to describe other ships of non-aligned nations. These ships are used for cargo and passenger missions. The designation is used as a general description as "Starship Class" is used as a general description by Starfleet to define the first U.S.S. Enterprise NCC-1701 a century earlier.

The Antares Class of Starfleet ships is retired by 2369. The remaining ships in the fleet, ex. U.S.S. Hermes NCC-10376, are sold into the market for use by civilians.

Andrew