Controlling Reinforcements? Need Help!

I gathered 2 huge armies and marched on a French City. I put it under seige with my strongest army and had my other army back me up. When I assaulted the city a had reinforcements right outside the battle area. I put it where i could control both of the armies and deployed my first. The other army never came into the battle field.:furious3: I had the option to WAIT, should i have? At about 3/4 of the way through the battle the general of the reinforcements came into the field of play but the rest of the army stayed outside. What should i have done?

P.S. I killed 1041 of 1200something of them. They only killed 400something of mine.:wall:

You know, instead of posting a new topic you could have posted in the topic below which has almost the exact title as yours and a lot of replies... I'll link you to it:

https://forums.totalwar.org/vb/showthread.php?t=73905