The Empire has three great advantage:
- Hypermatter annihilator: This technology provides magnitudes more power than the antimatter reactors in Star Trek.
- Hyperdrive is almost as flexible as warp, (allowing ships to jump without outer help and almost everywhere.) but almost as fast as transwarp. (crossing the galaxy in some days) And it looks like, there is no hypermatter without hyperdrive. (Wookiepedia)
- Since the GE controls almost the entire galaxy, it has significantly more resources than the Borg, only possessing the 1/8 of the Milky Way. (Asuming roughly same density of habitable planets and strategic materials.)
But the Borg wins in almost every other field:
- Transporter technology
- Adaptability and technological flexibility
- Morale: the Borg has no moral in conventional sense, but the Borg drones, controlled by the hive mind, would willingly sacrifice themselves for the Collective, while we see that Stormtroopers surrender against the primitive ewoks and Han Solo in Battle of Endor. The Empire is confident with its galactic hegemony, showing no interest in extragalactic expansion, leaving for the Borg, pursuing the perfection endlessly, to discover them, and choose the time and place to fight.
- Modular and decentralized vessels: GE ships appear to have Achilles heels, (Thermal exhaust port on Death Star, Shield domes, bridge and hangar on ISD-s) while a Borg Cube can continue fighting even if 78% of its structure gets destroyed (Memory Alpha)
- Sensor and cloaking technology: The Imperial sensors are primitive: The Millenium Falcon could hide on the hull of a Star Destroyer in SW V, and the Death Star couldn't detect it and warn it's fighters at Yavin. They also appeared to have no environment scanners on their shield projection base on the Endor, but was assaulted by the ewoks, and foolishly run into the trap.
So my scenario looks like this: The Borg constructs an transwarp conduit somewhere hidden in the Outer Rim, and sends probes to investigate the galaxy. Some probes encounter Imperial Navy vessels, but quickly self-destruct. After some months on research, the Borg choose a poorly defended, insignificant planet, and assimilate it with three cubes. Let's say it's defended by a small garnizon, and an outdated Victory-class destroyer. If they can beam through it's shields, the battle wont last long, but even if not, and the SD's turbolasers deal heavy damage to the cubes, they adapt their shields to minimalize damage, and destroy the bridge using the old combination of shield-draining beam and cutting beam. Then they tractor in the escape pods and the civilian vessels of the planet, and assimilate everybody, who they can. (It's possible that, some species are immune to assimilation, both the most humanoids (including the humans, making the majority of the Imperial military) can be easily assimilated.) Then they destroy all the clues and retreat into our galaxy.
Having ships in its hands, and some engineers in its collective mind, the Borg assimilates all the captured technology, constructing Hypercubes fitted with hyperdrive and hypermatter reactor. Then in a couple of years they conquer the entire Milky Way. They can attack the Earth (and all key Federation, Klingon, Romulan... planets) with hyperdrives, assimilating them, before the Starfleet ships learn about the attack and sprint home, rendering Starfleet to Rebel-like outlaws roaming space homelessly, and slowly dying without personal and material supplies. With this, the Collective had compensated all the three main advantage of the Empire, which is still unavare of the Borgs presence. And how knows, may they could get some thransphasic torpedoes and ablative armor on the Earth
Its highly likely, that on the first assimilated planet, the Borg had found some Force-sensitive people. There are many possibilities:
- Force makes immune to nanoprobes. Best for the Empire, allowing only macrosurgical assimilation, but the Borg still learned about the Force.
- The Borg can develop some Nanotechnology or bioweapon, rendering Midi-chlorians useless.
- They can propagate Midi-chlorains, making all the drones Force sensitive. (Worst for the Empire)
After that they can launch their attack to Corusant, Kuat, and all the Empire, saying 'resistance is futile'. While the most of Imperial weapons (from blaster pistols to turbolasers) don't have settings and modulations, the Borg can quickly adapt their shielding to them, and annihilate or assimilate everything they find.
Possibly, the Borg assimilates an Imperial Officer, knowing about the Death Star, and they can send back an captured ISD with it's original (but inflected) crew, to get it, but even if not, and they run into it, and the superlaser vaporizes several cubes, they have solutions:
- Kamikaze attack with a cube on hyperdrive or warp 9.99
- leaving Subspace on the shortest safe distance, on the oposite side of the station, than the superlaser emitter, and landing on the surface, before DS could roll and use it's main weapon. Then they can cut the hull, and send hundreds of thousands of drones to assimilate the crew.
- One gramm red matter from assimilated vulcan scientist.
