Within one generation give or take there was a revolution and said dictator ate a bullet.

The TFR is Romania is now about the same as everywhere else , 1.6 or so. This number give or take a few points is the birth rate of the developed world and one hopes soon to be the entire one till humanity voluntarily stabilizes at some lower number.

My opinion here is that its an issue of social carrying capacity, the more urban a society becomes and note the US is 80% urban, the more likely it is to achieve low fertility if its average IQ is 95 and up

This effects nearly every society, Brazil has the same TFR as the US, Mexico much of it rural hits 2.24 or ,maybe lower ,some say replacement and the rest of Latin America is transitioning to low fertility. slowly but surely

And note religion is irrelevant, Iran is a actual theocracy with German level fertility and Poland for example who just made Jesus king has a lower fertility that White Americans

In fact I'll argue that once a certain level of development is achieved, low fertility is inevitable . The US went below replacement in the 30's and stayed at that or at replacement till the baby boom

That boom was an anomaly caused by lingering pre development capital and war euphoria massive economic growth along with sudden declines in infant mortality

The boom ended in 1972 and only achieved a tiny bump one year 2007 do to a weird confluence of circumstances

The boom itself wasn't that high all things considered hitting around 3 and change on average (counting 1971 as the end year)

Its also not repeatable, changes in technology will ensure that 89% of workers will never receive significant wage increases in terms of GDP , there won't be another WW2 type war either that won't result in all societies being leveled and of course the old social capital is gone

In fact the only reason the US isn't even poorer is that 40% of the GDP is government spending mostly paid for by us being the reserve currency !

End that and the US GDP drops precipitously , leaving as as second tier through still developed nation only one with a crumbling infrastructure and incompatible populations

Long and short, human nature + development + cities puts a hard limit on the population .

As for the Johansen virus? I like blonds but why not brunets and redheads too?

Still ethno specific or tailored plagues and gene driven kill-squitoes would be much simpler or just use autonomous weapons systems to destroy targets food and water infrastructure

Trying to give all fertile age women baby rabies is silly even for John Ringo

Japan is such an outlier as well as a harbringer when it comes to procreation. I've taught many Japanese both men and women. Over the years I've picked the following up:

1)Women, especially the corporate workers deeply resent that they're regulated to overglorifed tea pourer. A lot are smarter and more driven than the men. In fact if they were allowed to be entrpreneurs or better yet momtreneurs, Japanese problems would resolve themselves
2) Post war Japanese men are such gammas that they radicate gammitude that even make the most radical feminist wince. Most are corporate drones who've literally become NPCs and have been so thoroughly conditioned that the majority have fallen to acedia (the spirtual vice of deep indifference)
3) The lack of risktaking and thus entrepeneuralism as well as the overemphasis of formal education has totally stunted that astonish creativity that Japanese have.

I have an even more radical solution: Japan needs to reembrace Christanity.
I'm inspired by Nassism Nicholas Taleb's Politics in complexity (it's a 12 point chart you can find in his Twitter feed) I find it very arresting and gives me a lot of food for thought. I think you'll greatly appreciate its insights
Being prejudiced, I'd want them to take up Catholicism. To me Japan's greatest tragedy was to reject Christanity. I really think Japan would similar to the Phillipines (i.e. a society that loves kids, treats women respectfully and everyone with dignity) but with the Japanese ethos. Can you imagine what a positive influence Japan would be?

No easy solutions.
xavier