Is that military men or does that also include women and children? A fair assumption would be that women and children were included in those ~40 million. How many or what percentage I cannot answer due to lack of surveys conducted at that time. But as far as I'm aware, If you opened your gates and laid down your arms, you'd be allowed to live while paying tribute to your new Khan. But if you didn't, they would operate mostly like everyone else. Kill every man in sight, kill their sons, kill some of the rest for good measure, let a few run away to tell the tale, and enslave the rest.

Every man able to hold a spear. Is going to hold a spear. When enemies are knocking at your gate. Every man is a military man. The concept of "Military" and "Civilians" is a very modern concept. There was no distinction between the two.

What do you consider a "long period of peace"?

War has been pretty damn constant throughout the ages. England and France had a war lasting more than 100 years. Famously called "The hundred year war". The USA have since their inception had roughly, 20 years of peace. Total. Africa has been a clusterfuck of warlords and infighting since (at least) the end of WW2. (Yes, I know why that's the case. But that's another topic for another day). The Middle East have always been good at war long before the brittish started drawing lines. And as for Asia. Well, we covered part of that with Genghis Khan. Not to mention the various Chinese dynasties and their conquests, infighting and rebellions. Japans recorded history is absolutely filled to the brim with wars.

Wars and fighting have been pretty much non stop for the vast majority of our history. The longest time of "peace" in Europe is considered to be either 1815-1914 or 1945 to 2022. But even those have the asterisk of "Peace among major powers in europe". Since there's been plenty of armed conflicts in Europe even during those years.