All neutral countries were guilty of helping a country out ok. Like imperious said history lesson forum go there. If you want house rules go for it i will allow nuetrals to be attacked and give them an ipc value and a certian amount of units inside each territory.
My example Spain they would have : 4 ipc then they would have somthing like 4-10 inf., 3-4 artillery, 2-3 tanks, 1 fighter (maybe 1 bomber but most likely not).
But neutral attacks bring it i will probably make sure that switzerland will stay neutral for ever because ill give switzerland (2000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000… and so on)