Haha. An army of women on their period certainly is a terrifying thought.

However, in seriousness, men have biological physical advantages that allow them to be stronger and have greater endurance. Bigger muscles, narrow hips that allow for faster movements. This stuff is real. This is not bragging, I'm just stating a scientific fact. There are many other areas where women are superior, but physical strength and endurance is not one of them. Therefore, at this moment in history women needed and relied on men to defend their land.

Hypothetically, if the allied forces in WW2 had been made up of women, and they went up against the Nazi army, made up of men, they would have lost.

As the military becomes more and more technology based, you could start to argue that women could do just a good a job as men. After all, if the fighting is done remotely via drones or robots, physical strength is a non-factor.