"They were too many for us. We killed them, but our young men died, too, and we had not enough young men to father our children, so we must stop fighting." (from Treasure Mountain, by Louis Lamour)
Some cultures have been essentially militaristic: they lived to fight, and not the other way around. The entire nation gloried in combat and warfare. The problem is that war is entirely destructive, in terms of both materiel and personnel. At some point the nation becomes exhausted. As an illustration, one study found that the male-to-female sex ratio in the German state of Bavaria fell as low as 0.60 after the end of World War II for the most severely affected age cohort (those between 21 and 23 years old in 1946).