Manpower is the stock of trained fighters that your nation has ready to send into combat. Each attack you make costs some amount of manpower, determined by your nation’s Manpower per Attack stat. The more manpower that goes into an attack, the more powerful and likely to succeed that it is, but also the faster it burns through manpower. Your nation is also continuously generating manpower, according to its Manpower Generation Rate stat. The faster a nation generates manpower, the more it will be able to attack before running out. There are many advances, as well as some resources, that can increase a nation’s Manpower Generation Rate.
The Manpower bar in the upper right of the screen shows how much manpower your nation has stored up. The bar fills up as manpower is generated and empties as it’s used up in attacks. The total amount of manpower that your nation can have stored up is determined by its Manpower Max stat.
If your nation is running low on manpower but you want to continue attacking, you can click (or tap) the (+) icon on the left end of the Manpower Bar, to buy more manpower by spending credits. After you’ve bought 100% of your nation’s Manpower Max worth of manpower in one day, you will not be able to buy any more until the next day.