2063: A Borg sphere travels back in time to this date, in order to assimilate Earth before humanity discovers warp drive. The Borg were defeated by the crew of the USS Enteprise E, which had followed the vessel.

Sometime prior to 2293: The Borg assimilate the planet El-Auria. Some of the refugees make their way to Federation space. Despite repeated interviews from Federation authorities at the time, the El-Aurians refuse to elaborate on the reason for their flight.
Their reason for hiding the existance of the Borg remains unknown, though in hindsight it appears to stem from an El-Aurian ethical principle similar to the Prime Directive.

Stardate 32611: Magnus Hansen, a Federation exobiologist, along with his wife Erin and daughter Annika, begin searching for a rumored species they call the Borg. They travel aboard a private ship, the (U)SS Raven, NAR-32450.

Stardate 32623-32629: The Raven encounters a Borg vessel near the Romulan Neutral Zone and devise a way to follow it back to the Delta Quadrant via transwarp conduit.

Stardate 32634: The Hansens are discovered and assimilated by the Borg. The (U)SS Raven is partially assimilated, and crashes on a planet in the Delta Quadrant for unknown reasons.

The existance of this mission was not generally known prior to the return of the USS Voyager. Almost every fact about it is shrouded in mystery. Even the Raven itself is somewhat of an enigma, registered as both a privately owned ship and a Starfleet vessel under
civilian command. How Magnus Hansen learned of the existance of the Borg is also unknown. At the time of the Raven's launch, some scattered information had been retrieved from certain El-Aurians. It is assumed by this report that such information somehow made its way to Dr. Hansen, sparking his ill-fated mission. All other records, including the proceedings of the Federation Exobiology Council for this time period, remain classified.

Stardate 41986 (2364): Several outposts in sectors 3-0 and 3-1 along both sides of the Romulan Neutral Zone, including Science Station Delta Zero Five and the Tarod IX outpost, vanish without a trace. Physical evidence suggests that the outposts were literally scooped off the surface of the planetoids on which they were located. Later examination of a planet in System J-25, which had been assimilated by the Borg,
revealed identical evidence.

Stardate 42761 (2365): The extradimensional entity known as "Q" transports the USS Enterprise D over 7,000 lightyears from Federation space, in the vicinity of System J-25. Sensor scans of the sixth planet indicated that every population center had been simply removed in a similar manner to that observed in the Neutral Zone incidnent. This is discovered to be the work of a Borg vessel. This marks the first official contact between the Federation and the Borg.

The Borg initiated contact by transporting a single unit aboard the Enterprise to access the ship�s computer. Following the crew�s attempts to halt the incursion, the Borg vessel attempted to assimilate the ship, having found its technology to be something desired by the Collective. Despite valiant efforts on behalf of the crew of the Enterprise, the ship was outmatched and would have been overtaken had the Q entity not again intervined, sending the Enterprise back to Federation space.

This encounter marks the apparent beginning of the Borg�s attempts to actively assimilate the Federation.

Early 2366: Starfleet organizes a Borg defense research project, led by Lieutenant Commander Shelby. They hope to devise weapons, stratagies, and tactics to be used in the event of a Borg invasion. Despite little data about the Borg, a number of new weapons are concieved of. However, the Borg arrive too soon for any of them to be put to use. Although the project was unable to provide much assistance during the first Borg incursion, its findings were eventually put to use throughout Starfleet, notably in the
Dominion War and the Second Borg Incursion.

Stardate 43989 (2366): The Borg invade Federation space. The first warning comes from the disappearence of the New Providence colony on Jouret IV. The Enterprise D along with Lieutenant Commander Shelby are dispatched to investigate. The site of the colony
is identical to the aftermath of similar Borg attacks in System J-25 and along the Neutral Zone. The next report came from the USS Lalo, NCC-43837 near Zeta Alpha II. The Lalo was subsequintly lost and presumed destroyed or assimilated.

Shortly after, the Enterprise located the Borg vessel. Instead of engaging in combat or attempting to assimilate the ship, the Borg beamed aboard and seized Captain Jean-luc Picard. Captain Picard was then assimilated into the Collective, becoming the drone known as Locutus.

By assimilating Picard, the Borg accessed Starfleet�s defense plans. But they also intended to use him as a spokesperson, in an attempt to intice the Federation to surrender. With Picard�s knowledge, the Borg were able to completely destroy the fleet assembled at Wolf 359, with virtually no damage to their vessel.

Fortunately, Picard was also able to provide eventual victory over the Borg. Following Wolf 359, the crew of the Enterprise made a daring rescue attempt, removing "Locutus" from the Borg vessel. Thanks largely to the work of Lieutenant Commander Data and
Commander Beverly Crusher, a way to communicate with Picard himself was discovered. It was this link that allowed Picard to relay information about the Collective to Commander Data. Specifically, he indicated that a command to send the Borg vessel
into standby mode might be able to bypass Borg security measures.

The attempt, of course, was successful, leading not just to a shutdown of the ship, but an overload of its power generators, destroying it.

It was in the aftermath of this first incursion that the beginnings of actual hard data began to accumulate regarding the nature of the Borg, thanks to debriefings of Picard, along with examination of the implanted technologies which had been placed in his body.

Stardate 45854 (2368): The USS Enterprise D encounters a crashed Borg vessel, believed to be a scout, in the Argolis Cluster. The ship had a crew of five Borg, only one of which survived. This drone, designated Third of Five, was brought aboard the ship due to the
urgings of CMO Commander Beverly Crusher. There the drone was brought back to health and examined. During the course of these examinations, the Enterprise crew
determined that, with enough study, an invasive program could be implanted in Third of Five, one which could ultimately bring down the entire Collective.

However, having been both removed from the Collective and exposed to an individualistic society, Third of Five began to assert himself as an individual. The crew went so far as to give the drone a name, "Hugh". Because of this, Captain Picard made the decision that to use Hugh as the instrument of the Borg�s destruction would be an unsupportable decision ethically.

Hugh was given the choice of staying aboard the Enterprise, but he chose to return to the Collective, stating that if he did not, they would pursue him. Hugh�s reintroduction to the Borg would later prove to have effects nearly as disasterous as the planned virus.

Captain Picard was eventually repremanded for his decision, though no official action was taken as many in Starfleet Command tended to agree that the decision was too ambigious for a clear judgement to be made.

Stardate 46982 (2369): Contact is lost with a Federation colony on Ohniaka III. The USS Enterprise D is in the area, and discovers that the colony had been attacked. An away team eventually encountered and traded phaser fire with a group of drones. However, these drones were extremely unusual. Aside from showing no interest at all in assimilation, they refered to each other by names. The Enterprise then engaged a Borg vessel of new design, which escaped by beaming over several drones as a distraction. One of these drones, which called itself "Crosis" was captured and placed in the brig.

Soon after, the MS 1 Colony was also attacked by these Borg. Panic quickly spread throughout the Federation, leading to multiple reports of Borg attacks where no Borg were present. Starfleet, attempting to sort out this situation, initiated a level-2 security
alert fleetwide.

Eventually it was discovered that these Borg where what was left of the branch of the Collective that the drone known as Hugh had returned to. His individuality somehow infected his fellow drones, leading to them being cut off from the rest of the Collective. They wandered aimlessly until encountering a being they refered to as "The One". This being was later found to be the Soong-built android Lore, part of the same series that had produced Lieutenant Commander Data. Lore was using these Borg as a way to strike back against the Federation.

Lore conspired to kidnap Commander Data, which led the Enterprise to his headquarters on a planet of undetermined location. Lore was overthrown by the Enterprise crew, with help from Hugh and a small group of Borg who remained skeptical of Lore�s claims to be
working for their benefit.

This encounter marked the first indication that the Borg use transwarp conduits. At first it was unclear whether the conduits were something Lore had developed from existing Borg technology or belonged to the Borg themselves. Later findings reported by the USS Voyager show that the conduits are indeed Borg devices, along with a host of other transwarp capabilities.

Hugh and the rest of the now free former drones remained on the planet to form their own society, free of outside influences. Unfortunately, the transwarp conduit which leads to their world is no longer accessable, making their current status unknown.

Stardate 50541 (2373): The USS Voyager discovers the corpse of a Borg drone on an unnamed world in the Delta Quadrant. The native inhabitants, called the Sakari, had been almost entirely assimilated by the Borg, leaving only a few handfuls of survivors. They had avoided detection by fleeing into a network of mines that contained sensor-scrambling materials.

Stardate 50893 (2373): The Second Borg Incursion. It begins with an attack on Ivor Prime. Starfleet quickly organizes a fleet to engage the single Borg vessel in the Typhon Sector. Despite his experience with the Borg, Starfleet Command decides that Captain
Picard�s history with the Borg makes it unwise to send him to face them. Picard and the USS Enterprise E later break with Command�s orders upon hearing that the defensive grid in the Typhon Sector was quickly broken by the Borg ship, which was once again making its way towards Earth. The Enterprise E was able to rendezvous with the rest of the fleet, which had been pursuing the Borg ship with little success, in the Sol System. Once there, Captain Picard experienced moments in which he was aware of the thoughts of the Collective. This had been the original fear of Command, and the reason the Enterprise was ordered to remain out of the battle. However, rather than being brought back under the Collective�s control, Picard was able to learn of a momentary weakness in the Borg vessel. A weakness the fleet quickly exploited to destroy the ship.

Shortly before the Borg ship exploded, it launched a smaller vessel. It was this ship that generated a temporal rift and traveled back in time to the year 2063 in order to stop Zefrem Cochrane from making his first warp flight. As mentioned earlier, the attempt
was twarted by the efforts of the Enterprise crew. Upon the Enterprise�s return to the 24th century Starfleet was able to examine the large amounts of Borg equipment left behind on the partially assimilated ship.

Stardate 50614 (2373): The USS Voyager encounters a disabled Borg ship in the Nekrit Expanse, a region in the Delta Quadrant filled with plasma storms and various unknown types of interstellar gas clouds. The ship and the Borg aboard it were offline, but a group
of drones who had been on a nearby planet found themselves free of the Collective�s influence, and their original personalities began to reassert themselves. Among these former drones were several residents of the Alpha Quadrant, including at least one
Starfleet officer present at Wolf 359. It is our conclusion that the Borg sent back a sample of assimilated beings sometime after Wolf 359, possibly through a transwarp conduit. The reasons for such an act are not known, though it is hypothesized that it was an attempt to ensure that some data on the Alpha Quadrant would be preserved even if the Borg invasion of Federation space failed.

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"And much of Madness, and more of Sin, and Horror the soul of the plot."
--
The Conqueror Worm, by Edgar Allan Poe

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A-"Dippidy Doo." Q-"What forms on your dippity early in the morning?"--Johnny Carson

Good job Sol.

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"One Tequila, Two Tequila, Three Tequila, Floor". George Carlin

What I think is much more likely is that the El-Aurian refugees simply didn't know what had happened. We don't know where El-Auria is, it could well be in the DQ and the refugees had been travelling for some time (we know they are long-lived) and had only just arrived in the AQ immediately prior to ST:G (unfortunately running straight into the Nexus). The group on the transports might well themselves be colonists, they didn't escape from the El-Auria itself and never witnessed its assimilation but only received garbled messages about vast cube shaped ships and half organic-half machine creatures which called themselves the Borg. The group realising that their colony might be next of the list, pack up and leave. By accident they avoid the Borg completely and after wondering across the galaxy arrive at the Federation. They tell their (unverified) story (which isn't very much) and it simply gets filed away. No cover-up, no plot, no attempt to avoid panic-the whole thing is simply regarded as a garbled, second hand fairystory.

Except for a few scientists, including one Magnus Hanson who is seventy years later is convinced there is more to it. He badgers the Federation Science Council to authorise an expedition which eventually they do. (I wonder if he may be related to Admiral Hanson of BoBW fame and used a bit of nepotism). Off Magnus flys dreaming of vindicating his theories about the fate of the El-Aurians and is never seen again. The Raven is known to have crossed the neutral zone in violation of instructions and is eventually listed as missing. I victim of the many dangers of deep-space flight. The rest is history.

Simple and I think completely consistent with what we know.