My first comment is that, looking through the MSN convo, the entire Death Star analogy was just silly.
The Death Star was destroyed due to a dumb mistake on the Imperials. The cube was destroyed due to the Enterprise getting Picard. That much is true. But theres one difference- there was only one Death Star, but there are hundreds or thousands of cubes.
A better analogy would be if the Empire sent one ISD to destroy a Rebel base, and the ISD was damaged in the attack (as the cube was damaged at Wolf 359, but regenerated afterward), but a Rebel Saboteur blew it up.
Would the Empire sit, give the Rebels a few years to prepare, and send ONE Star Destroyer? NO! They would IMMEDIATELY hit it with another ISD, or more likely, two. Yet the Borg gave the Federation plenty of time (years) to prepare, then sent the same amount of force as last time, which got it's rear handed to itself.
And don't say that Picard was unanticipated- if Picard is still partially connected to the Borg, they'd know as well as he does.
Okay, now that that's done with...
-The Borg send another cube in FC. Given the extremely implausible nature of how the first cube was lost, sending another single cube makes sense. The ease by which the first cube dealt with previous forces suggests even stronger forces could be safely dealt with by a single cube. And they were(Starfleet forces were dropping like flies), right up until the cube virtually entered Earth orbit.
This would make sense if they did it right away. If you're going to give your enemy years to prepare, with them knowing you'll likely attack again, you're going to need more forces, since they'll be ready for you.
After all, the first cube got the drop on the Federation- they were NOT prepared. By the time of the second cube, they had spent YEARS preparing, building ships like the Defiant, etc. Obviously, the Borg should have predicted the Federation would not sit around idly, and either sent a cube IMMEDIATELY, or sent two or three after a few years.
Fast forward to STVOY "Endgame", and we find out the Borg have created a Transwarp conduit less than a lightyear from Earth. Starfleet forces would only be given minutes warning time, and deploying multiple vessels virtually at Earth's doorstep would be exceedingly easy. It never happened, thanks to Voyager's time travel copout, and the Borg probably would not have initiated this attack any time soon regardless (recovering from the massive Species 8472 war would take priority over assimilating a low priority target halfway across the Milky Way galaxy)
Wow...the Borg have dozens of cubes constantly going through this transwarp hub daily, and one of the conduits goes STRAIGHT to Earth, and they never bothered using it?
Seriously, all they had to do was sent TWO of the several cubes we saw defending and travelling through the transwarp hub through that conduit, and Earth would be gone. Did they? No. They were giving humans yet ANOTHER few years to prepare.
Let's see what YT replies
