Need citation for this. War is a net negative every time. War destroys resources and kills people. This leads to a labor shortage. It also destroys property so it leads to housing crisis and famine.
Except that raising children requires more time and resources than caring for elderly. So having less children frees up more resources to care for the elderly. Into the next generation there are now less people which require even less resources which means you need fewer workers to produce those resources.
That is a death spiral. You can consider the labor involved with caring for the elderly a sort of tax on labor. It’s a net drain but required and is directly related to previous generations of labor. The labor involved with raising children is similar but is closer to an investment. The more labor done for raising children, the more labor there will be next generation. Even though the labor for children is higher than the labor for the elderly, it results in a net positive.
If you have vastly fewer children in the following generation, you end up with a higher percentage of elderly labor compared to the labor pool. If the labor for children goes down enough to more than make up for it, you don’t have a per capita labor deficit. BUT you do have less total labor.
Now we get into the real issue: maintaining society. It isn’t just about the labor to care for each other. But technology, infrastructure, food, etc all need a certain amount of labor. And most of these tasks are scalable so it requires less labor per capita as population increases. If you shrink your labor pool too quickly, you won’t be able to sustain your infrastructure causing a collapse.