From now on, your warriors will swear oaths of allegiance upon completion of training. They will no longer abandon you in times of hardship - a Spartan’s vow is unbreakable.
When you have less Grain than your Units require, they shall no longer die or desert you. You can use your Resources to train all the Units you need, and they’ll never leave you for lack of Grain.
Remember that Units still consume Resources, so monitor your Resource Production. You cannot have less than 0 Resources, but your Units may use up Resources faster than you can produce them. Prevent this by constructing and upgrading all Farms and Granaries!