If you kill 100s of thousands of your child bearing females in war, where do your future generations, and future military come from?
Now you are catching on.
The total US fertility rate is less than replacement levels of 2.1 (as of 2011 it is about 1.89). You have women too busy working to have children, women have unlimited abortion, pets have become substitute children, plus having women in the military doesn’t help matters.