Looks like I just made the same mistake as Napoleon. I though my armies are soo big it means nothing if I lose some to attrition. Well, my pop though otherwise.It is possible, some of middle eastern provinces(deserts) have very low supply limit, thus not allowing you to field a large army there.If you are playing on a political map, you can actually see the supply limit of every province at a glance if you have an army selected.It could also be possible that some of your armies are sitting in foreign territory where you don't have military access.