With a number like NCC-1764, you would think that U.S.S. Defiant was a relatively late launch, maybe from the 2280s or so. However, this ship is actually part of the original Constitution class. After issuing sequential contracts for 1700-1711 (plus the two refits of 1017 and 1371), 1764 really stands out as an anomaly. The answer may lie within the Kelvin timeline, however. In that universe, which diverged from the Prime timeline in 2233, 1764 Defiant was constructed as a Mayflower type cruiser, and shared many gross spaceframe characteristics with the planned Saladin interceptor that went on to become the Class 1 Mk VII hull. It's possible that Defiant is in fact a third "X to Connie" conversion like 1017 and 1364, adapting an in-progress Mayflower build into the Class 1, Mk IX configuration.

Star Fleet Battles concurs that Defiant was an upgrade from an earlier cruiser design, though it lists NCC-1615 as the ship's hull number. Curiously, along with Defiant, a number of other previously documented ships in this range, including 1616 (Zimmerman, Helios class fleet tug) and 1619 (Miyamoto, Malachowski class light cruiser), share a similar configuration with the Mayflower class, it's possible that 1764 is actually a replacement for an earlier 1615 and the Star Fleet Battles historian conflated the hulls. 

Regardless of the exact details of Defiant's origins, it appears she launched some time around 2245 or 2246, consistent with the earliest Constitution class ships. In this era, she would have been equivalent to or slightly more capable than a the Star Fleet Battles YCA configuration. It's likely she had warp-targeted lasers rather than phaser batteries, though these would be late-model versions that are probably comparable to what Star Fleet Battles classifies as a Phaser 2. Despite her relatively early build date, sources record her as the 13th Connie to launch, which suggests a delay of up to 2 years between her launch and her final commissioning. This might indicate "teething problems" that had to be worked out before she was ready for service, or it might just point to a longer than normal shakedown cruise.

After joining the fleet, Defiant's initial missions involved patrolling the Klingon Neutral Zone. Relations with the Klingons had strained considerably in the 16 years since the "diplomatic war" of 2234-2235, and with rumors reaching the Federation of an uprising in one of the Klingons' vassal civilizations, Military Operations Command adopted a strong deterrent posture at the border to ensure that any future disputes went to the diplomats before "face-saving" military action breached the Federation's borders. It didn't take long to realize that the deterrent displays on the border had failed to achieve their goal; instead, they'd just driven the Klingons to explore other tactics. 

We've mentioned before the fundamentally porous nature of the Federation border. While the Klingons were forced to "acknowledge that the Federation claimed" the border zone with the Klingon Empire that the Federation Council defined in 2229, they had never accepted it - in no small part because it contained a number of star systems that the Klingon Empire had controlled in past historical periods. Rather than confront Federation patrols directly, the Klingon Imperial Fleet simply used longer-ranged vessels to bypass border systems and look for strategically important footholds in the vast unexplored areas of the Federation's claimed territory. In the end, they chose Axanar.

Axanar, in the Delta Orcus system, was an underdeveloped planet fairly deep in the Federation frontier, and one of three different planets to share the same name and provide a home for very similar species. All of those species, in turn, bear some genetic similarities to the Klingons. While even the more advanced Axanarri of Toredar and Epsilon Indi have only fragmentary records of their pre-industrial histories, some scholars suspect that the Axanarri may have been a former client race of the so-called Old Kings, who also counted the Klingons as vassals in the period from approximately 1967-2140. 

Because of uncertainties involved with translating the Spaceflight Chronology reference stardate system to contemporary dating, we don't know exactly when U.S.S. Gulliver discovered the Klingon presence at Axanar on stardate (1/) 9404.1, but this date likely fell on the earlier end of a range between 2246-2251. Indeed, if this encounter - and the Four Year War that it touched off - occur before 2248, then Defiant's initial mission may have been a "hot" patrol rather than mere deterrence... or alternatively, the claims that Defiant was the 13th Connie to commission into the fleet are simply wrong... or some of her early action actually occurred during her shakedown cruise. Any of those explanations are plausible. 

Somewhere in the range of 2247-2252, depending on date conversion, Defiant worked with a squadron of older Baton Rouge class cruisers counter-ambushed a Klingon attach on a large Federation convoy in the Rebonet system. The cruiser force destroyed or captured all eight of the light cruisers and gunships sent against the convoy. Historians note that this was the first time when Federation tactics gave them a qualitative advantage over similar-strength Klingon naval forces, which is probably a reference to the "Fighting Instructions" which generally called for a strong first strike using pre-armed "hot" photon torpedoes and phasers, followed by formation maneuvers at 1/3 impulse power using only phasers for firing, or slowing to 1/4 impulse while charging and delivering concentrated photon torpedo attacks from multiple ships against individual targets. This remained Federation tactical doctrine until the Klingon War of 2256 and remained in use with task forces that contained under-powered ships such as the Saladin class for many years afterwards.

A ship-related side note about the Four Years War period - the FASA sourcebook for this period indicates that the Achernar and Tikopai classes were introduced during this period. This is almost certainly an error. However, the introduction of the Bon Homme Richard class generally lines up with this period and that's probably what FASA means when they talk about the Achernars. The Tikopai reference may be talking about a completely different ship than the Tikopai from Ships of the Star Fleet, but if 'Achernar = BHR' then it's not unreasonable to say that the references to the Tikopai class are actually references to the Sombra class. 

In approximately 2255, Defiant received a major upgrade. FASA indicates that this upgrade brought her up to Achernar standards, but that's about ten years too early, so more likely this was a refit to the Bon Homme Richard (GCA equivalent) configuration, including the phaser refit that Enterprise got a few years later. During the Klingon-Federation War of 2256, Defiant was assigned to protect Sector 006 in the Federation's interior; this sector contained, among other things, the Procyon star system and the Black Star (though the latter wouldn't be discovered until 2267). It's likely that Defiant avoided major action during this assignment, but she may still have had to contend with raiders and other light, long-range threats.

In 2260, Defiant formally started her first five year mission. This may indicate that she was previously assigned to Military Operations Command or another major command within Starfleet and transferred to Galaxy Exploration Command at that time. Some time before 2265, Defiant received another upgrade, this time to the Achernar standard. Like Enterprise, she did not have the command cruiser package, and would be a CA by Star Fleet Battles standards. In 2265, Captain Serling handed over command to Captain Thomas Blair, who started a second five-year mission in early 2266.

The second FYM saw Defiant operating near the Klingon Neutral Zone and notably in the Taurus Reach, where the Federation was conducting a colonization and exploration campaign. In 2267 she clashed with Klingon forces in the Golmira system, then a year later breached the Neutral Zone to respond to a distress call from the Klingon colony on Traelus II. They arrive to find no survivors but do find signs of Tholian presence. Shortly thereafter, while returning to Federation space, Defiant detected several Tholian starships in pursuit and attempted to escape before becoming trapped in a spatial interphase.

Highly sensitive records only recently declassified by the Department of Temporal Investigations reveal additional details: time travelling Na'kuhl forces, seeking revenge on the Tholians for the future destruction of their home star, were attacking the Tholians in Defiant's time, prompting the strike against Traelus when the Klingons were misidentified as the culprits. Defiant's efforts to outrun her pursuers ended at the Murasaki 242 protostar nebula, when she discovered that she was not being chased... but rather, the Tholian ships were responding to a distress call from a Tholian colony fleet that was under attach by the Na'Kuhl. 

Defiant, and an unidentified Pioneer class ship intervened to save the colony ship, but not before destruction of the Na'Kuhl command ship triggered the interphase event and trapped Defiant. For reasons that remain classified, the Pioneer's captain could not or did not assist Defiant. What's publicly known about this incident leaves many questions unanswered. All other records indicate that the Pioneer class ship present at the incident was actually deployed to a completely different part of the Federation. Moreover, some temporally shielded data repositories record a different class of ship being present in the incident, most frequently a Constitution class ship. The Department of Temporal Investigations has refused to comment on these discrepancies.

Defiant remained trapped in interphase until, responding to her distress call, U.S.S. Enterprise attempted to rescue her on stardate 5693.2. Unfortunately, the Tholians interfered in the rescue attempt and Enterprise was unable to retrieve the trapped ship. By this time, the crew of Defiant had already perished, and Starfleet wrote the ship off as lost. At some point after this rescue attempt, Defiant was independently temporally displaced to at least three different temporal-spatial coordinates.

One Defiant was transported into the so-called Mirror Universe, a tightly intertwined quantum reality that is 'coiled' with the Prime timeline. This Defiant led to an unintended technology transfer to the Terran Empire, greatly impacting events in that universe. The official position of the Department of Temporal Investigations is that this outcome is an intended part of the Prime timeline's temporal topology and is not in fact a temporal contamination incident. A second Defiant was recovered by the Enterprise (its NCC-1701-A version) in 2288 utilizing experimental phase inverter technology. At the time, the Defiant's presence in the Mirror Universe was a closely guarded secret and the general public widely believed that this recovery operation closed the book on Defiant's history.

However, in 2298 the U.S.S. Excelsior briefly observed the Defiant while traveling through a spatial rift in Tholian space. While they initially assumed that this might be the same Defiant that Enterprise had recovered ten years earlier, the same "ghost ship" was observed by the Tholians in 2376*. U.S.S. Da Vinci finally recovered the third and last known interphase duplicate of Defiant. I add an asterisk because this final encounter occurs after the temporal fracturing event of 2373 and may not have occured the same way, or at all, in the Prime universe. Unfortunately, the successful use of the CODA device means that data from the Splinter Universes and their Mirrors is often incomplete or missing critical contextual data to fully analyze its parallels with the Prime universe. 

In both of the latter cases, we can safely assume that Defiant was either taken apart to fully understand the effects of long exposure to interphase, became a museum ship, or some combination of the two, as she was already obsolete in 2288 and was a literal museum piece in 2376.

There are, of course, an infinite number of different universes that make up the multiverse. One quality that varies from universe to universe is stability - some universes appear to persist effectively indefinitely, spawning an infinity of parallel universes in response to quantum uncertainty and essentially acting as a 'backbone' for the multiverse. One of these is the Prime Universe and its entwined Mirror Universe, in which Star Trek takes place. However, in 2373 the Borg inadvertently created an unstable artificial parallel universe as a side effect of efforts to protect themselves from destruction during the so-called First Contact incident. Every Star Trek novel and video game set after 2373, and published before 2020 (plus the direct sequels to those products) takes place in the Splinter Universe. The events in those sources might occur in the Prime universe, but they also might not. 

To address the irreconcilable conflicts between these sources and Star Trek: Picard (as well as Star Trek: Discovery, Seasons 3+), the franchise conducted a partial reboot of the chronology. The circumstances that led to the creation of the Splinter Universe set in motion a series of events that could have led to extra-temporal life forms known as the Devidians ultimately collapsing every timeline, including the Prime and other hypothetical backbone timelines, to harvest an infinite supply of neural energy and therefore provide themselves with an infinite supply of sustenance. The protagonists of the the Splinter Universe set out on a series of adventures to stop the Devidians, ultimately intentionally collapsing their own timeline with a device called the CODA to permanently prevent them from accessing the Prime timeline.

This lays the in-universe explanation for the many cases where Picard explicitly contradicts events that happened in the licensed fiction, and requires very skeptical analysis of anything that occurs in those sources. In general, when there's a conflict with the canonical sources, it's safe to assume that the earlier version never happened or happened in a radically different way that's compatible with the canon. A partial exception is Star Trek Online. STO's backstory book, The Needs of the Many explicitly places it within the so-called "novelverse" chronology, referencing many events and characters from those books. However, in the Season 10 episode "Butterfly", the Young Admiral/Dahar Master/Honored First causes catastrophic, irreversible changes to the timeline through the use of a Krenim temporal weapon ship. Given that STO's story continues past the release of the Coda trilogy and incorporates events from the new canonical sources including Picard, it is safe to assume that the STO universe has separated from the Splinter Universe and stabilized, perhaps even becoming a new backbone universe in its own right.

The Federation commissioned a new Defiant using the same hull number in about 2271. This ship was an Enterprise mk I (CA+) heavy cruiser and also had NCC-1764 as her hull number. We don't know much about her early life, but she went in for a refit in about 2288. Normally I'd assume that this was a CX refit, but I have a different suspicion here.

As you probably recall from the Battlecruiser discussion yesterday, 1764 is also associated with a battlecruiser hull. I oversimplified a little in that case, however. 1761-1763 are known battlecruisers. For 1764 however, to quote the Federation Master Ship Book, some time after Y183 (2285) "The second battle carrier, Atlantis, was either completed in or converted to this configuration." The Atlantis name is a little challenging, however, because NCC-1784 was Atlantis before she was changed to Enterprise A just before commissioning. Since that occured  in 

2286, Atlantis can't be the second battle carrier. My guess is that the original plan was for 1784 to be converted to the BCS configuration, but when she was chosen as the next Enterprise those plans were delayed. Instead, Defiant II received the refit. 

The Battle Control Ship (contemporary sources probably called it a "Tactical Control Ship") was essentially a scaled-down Space Control Ship that combined a cruiser with a light carrier and a runabout tender all in one hull. During the refit, Defiant II's engineering hull was probably completely removed in favor of a heavily modified "Super Balson" design. She likely retained her saucer largely intact, with only relatively minor alterations. The engineering hull had a full-length shuttle deck that could allow for simultaneous launch and recovery of shuttles; normally, she carried ten shuttles, six of which were high performance fleet defense shuttles capable of both offensive and defensive operations. Additionally, she had six semi-external landing bays for runabouts. 

Operating in a task force, generally with a light cruiser and a tactical escort, Defiant II and her support ships could effectively control movement throughout a star system. In peacetime, she would have been an exceptional ship for surveying uncharted areas such as star clusters where the ability to 'visit' many different locations simultaneously would greatly speed up the mapping process. Her power projection abilities were also superior, hence the 'tactical control ship' moniker, and when called upon her task force would have been a close match for the capabilities of a heavy carrier group. 

We don't know much about Defiant's operations after her BCS refit, but it's unlikely she was kept idle. By 2287, the Klingon border had settled into a cold war stance, and the Organians had intervened to bring an end to conflict on the Romulan front. At the same time, the Interstellar Concordium was starting to deploy ships to 'helpfully' enforce the various neutral zones and avoid a resumption of hostilities. Early on, Defiant was likely responsible for supporting humanitarian efforts in the star systems that had been damaged in the various clashes of the 2280s, but after the first Andromedan attacks on the federation in 2292, that season of rest would be over and Defiant would surely have moved to participating in the Federation's response to the crisis. 

Around 2296, Defiant had another refit. We don't know to what exactly, but if I had to guess it would be to the HVX configuration. Like the Enterprise-A this was an "X-Technology" configuration with improved performance across the board. It further expanded the tunnel deck and moved the runabouts "indoors", displacing the fleet defense squadron and turning the ship into a pure runabout carrier. At the same time, Defiant's task force was redesigned to support a more nimble operational tempo, removing the light cruiser and instead pairing Defiant with either two escorts, two frigates, or one of each (historical records are unclear). 

Defiant II's career finally ended in about 2324, when she was decommissioned and scrapped. This probably made her one of the last of the Connie era ships to exit active service. In the years that followed, Defiant would return in 2370 as the prototype of a new class of tactical escort originally designed to fight the Borg (NX-74205). A fourth Defiant (ex-Sao Paolo) would succeed her in 2375 after the original's destruction. Those two ships have their own lengthy history, which is outside the scope of this article (and much of which happened in the Splinter Universe and isn't reliable source material - see earlier footnote).