Also as others have stated, just the shear scale of the task they jumped into, perhaps without as much thought as they should have given the matter. Once you start a war it does have the ability to take on a life of its own. As the various forces involved act and react against one another. In some cases success can be almost if not more dangerous then defeat. If it puts you in the wrong position, or forces you to do something you didn't take into account.
The headlong charge at Terra, complicated matters as well. Which ever clan claimed Terra first became master of all the other clans. Thats a hell of a lot of policitcal and military pressure to put on commanders in the field. Add the fact several of the clans involved in the invasion cannot stand each other ... well now you have a race that there is no tape to break.
There is more I can add to this, but its going to have to wait. I dont do my best thinking at 4:01 am in the morning
