Really? He runs off, abandons his forces, his forces are too dumb to survive without him, they lose.....
It was one of the worst transitions I have ever seen. It was lazily thought out, badly implemented, and killed any realism the series had. If they didn't have that transition it would have been one of the best anime ever made. Unfortunately they made that HORRID transition.
Also I was watching code geass the day it was translated after the jap release. I have been a huge fan of the series except that transition <_< soooo bad. So bad.
The reason they lost is because first of all, they were less in military might, it was lelouches intelligence that got them winning but think about it like this: What if the leader who they depend on suddently disappear? They will have a hard time with the opposing forces and if they lose a friend or someone close(which they did) then they will lsoe their cool which will be the end of them, they lost their cool and once they lost their moraly and trust then it was all over.
Lelouche put all the burden on one man to lead when he left and that man did the best he could but it was all sudden and the lives of the ones lost was his responsibility.
I don´t see how them losing affected the story´s realism.
Also don´t forget that the opposing force also had stronger knightmares,more people and most important, they had people who were on the same lvl of intelligence as lelouch and once he´s out of the picture, there is no way they can win.