The VE's inability to get out of its own way. The Vast Empire fell because it wasn't vast, and barely an empire. The VE fell because the leadership was removed from the day to day functioning, the administrative tasks, required to keep an empire going. It would have been nice, easy even, if another force had pushed it to the brink, but mostly, that didn't happen. What happened was the VE overreached and supply lines fell apart. The EEC did what it could to compensate, but it wasn't enought. At the same time, the DJO members had to retreat to fight, and mostly lose, their own war.
We lost, we failed. We can't change that, but perhaps we can find and bind what remnants there are together, to start again.