About Legion Stamina : Rise Of Empires
Each Legion or army that you have has an amount of Stamina that is shown in the Legion list screen next to a small hammer icon above each legion listed. The maximum that each legion can have at any time is 100.
Every time you march your legion to attack or gather it costs a certain amount of stamina so the number will go down. If the cost of a march is more than the stamina you have left it will not let you do it. Think of it as energy needed by the troops and that they need to rest to recover.
The stamina recharges over time, but if you are attacking enemies then you will use it up far faster than it regenerates. To let it regenerate you just have to wait a while to let it increase again. There is research you can do in the individual Legion research stems that improve the speed of the durability recharge, but there will still be a wait for it to go up before you can use that Legion again.
It is possible to regenerate the stamina faster with Stamina Potions. Using these will immediately recover a fixed amount of stamina to allow you to march the Legion again. However you will need to buy these in the store for real money. They are not available to buy for gems.