A culture whose women are the ones who go off to guard their nation and fight the wars, while military men of that culture are nowhere to be found, is a culture whose women are not giving birth, something only women can do, and raising families. That is a culture set to wither away and die within a few generations. Each generation of that culture would be a fraction of the last, and other tribes would quickly out-compete and overwhelm it.
No tribe that followed that insane "women are our war fighters" strategy ever survived past the stone age. Feminism is all well and good, but there really is such a thing as human biology.
What if only a small percentage of the women went to war in each generation. Thus most of the women in each generation would stay at home and be available to have children.
In some countries all the men have to serve a few months or a few years in the military to get military trainng, then they are in the reserves for a period of years and liable to be called up for military service in time of war. And some of those countries get in a lot of big wars and so military service consures of significant percentage of the time the men could have been doing civilian stuff. And other countries with unversal consription fight very few war and very small wars, so the percentage of time their men serve on active duty is small and they have lenty of time for civilian stuff..
And if the second type of country had universal conscription for women instead of men, the miitary trainng woould cut down on the women's chld bearing years a lbit. But so long as there weren't long wars involving large numbers of women, they still should have been able to reproduce enought to maintain and increase the population.
Or think about the UK during most of the period from 1815 to 1914. Military service was almost entirely voluntary in the small regular army. Except for the Crimean War in 1853-56 and the Boer War in 1899-1902, Britain didn't have a military manpower problem, despite fighting one or more colonial wars all the time during that period. At anyone time no more than about 1 percent of the male population of the UK was in the military which numbered only a few hundred thousand at its highest. If the UK soldiers had all been female instead of male, it would not have reduced reproduciton rates much.
In the 19th century, 1800-1900, the USA fourght 8 large wars, The War of 1812 in 1812-15, the Mexican-American War in 1846-48, the Civil War in 1861-65, the Spanish American War in 1898, and the Philippine-American War in 1899-1902.. In those four wars the majority of US troops were United States Volunteers. In the Civil War the number of men in the armed forces reached several percent of the adult male population, though well under ten percent, and the northern civilian farming and industry sectors managed to increase their output during the war.
But the USA was involved with smaller military conflicts in almost every year of the 19th century, on a somewhat smaller scale than the UK. - most of those conflicts with various nations, tribes, and groups of Indians within the US borders, though some with foreign countries like Korea. And most of the US armed forces in those conflicts were members of the regular army, the United States Army proper.
Throughout the 19th century the total of all the armies of the United States of America, including the United States Army, the United States Volunteers, and perhaps sometimes including militia called into service, varied from a low of 2,486 in 1803 to a high of 1,000,692 in 1865. During times without major wars, but constant minor conflicts, the regular army of the US that fought those conflicts never exceeded 30,000 men except in 1866 to 1869, and never exceeded 20,000 men except in 1866 to 1899. The regular army reached a maximum 19th century strength of only 57,194 in 1867. With the Navy and Marines, the maximum peacetime strength of all US armed services was only 76,749 in 1866.
http://alternatewars.com/BBOW/Stats/US_Mil_Manpower_1789-1997.htm
So if the United States armed forces had consisted entirely of women and girls instead of men and boys (with a few females disguesed as men) during the19th century it would not have been a significent drain on the availability of women for reproduction.
The idea that all societies everywhere and throughout all time fight so often, and with such a large percentage of their populations as warriors, that a military composed of females would always result in catastrophic lack of reproduction is a a gross exaggeration.