thaluikhain said:
It hasn't happened because no country can sustain the amount of casualties to make it necessary. You get conquered long before you lose enough of your population in front-line combat to make that sort of demographic problem.
The US has more military personnel than most, and it's still little more than 1.5 million out of 315 odd million people. If all of those were women, and all of those were front-line soldiers and all of them died, that's less than 1% of the women in the US.
It's because the US hasn't fought a total war since World War II, and even that wasn't particularly harsh on them compared to the other belligerents.
Army sizes have actually shrunk dramatically over the course of the 20th century. They spiked upwards when conscription was introduced during the French Revolutionary Wars, and continued to increase until WWII, after which point they began to climb downwards. For comparison, the US has 1.5 million soldiers today. In WWI, they mobilised nearly five million men, Britain eight million, Russia twelve million and Germany
thirteen million. And this was when the populations for all four countries were much smaller than they are today.
If you want an example of how total war can impact population levels, look at Russia. They lost
thirteen percent [http://en.wikipedia.org/wiki/World_War_II_casualties#Total_deaths] of their population in WWII. So yes; you can in fact fight a war for long enough that population loss becomes a serious problem without either nation being conquered. It's just never happened to America; the closest they got was the civil war, when they lost about two percent of the population.