“Spengler” offers a radical answer, which is plausible at least for the developed world.
“The personal is political,” said the feminists of the 1960s. They were on to something. Countries go to war because those who inhabit them cannot bear their individual lives. Entire cultures die out because the individuals who comprise them no longer wish to live, not because (as author Jared Diamond claims) they cut down too many trees. Bulgaria and Belarus have plenty of trees, yet we observe in such countries a demographic catastrophe unseen in Europe since the Thirty Years’ War.
Birth rates rise and fall with religious faith People do not have babies because religious doctrine instructs them to procreate, though, but because religion makes them happy. With the end of traditional society, religion becomes a personal, not a communal, matter, and the fate of nations is fought out at the level of individual souls. Communism suppressed religion in Eastern Europe, and the demographic data in consequence seem to bear out the cliche of the melancholy Slav. By mid-century most of the Eastern European countries will lose 20-40% of their people and be left with a geriatric remnant.
US Christians, by contrast, have one of the highest birth rates in the West. Conservative, mostly evangelical Christians have a plurality, soon to be a majority, in US politics. Their burgeoning power stems from a personal message that has made converts of tens of millions of liberal Protestants. Evangelicals are political only when circumstances force them into politics, for example proposals in several US states to legalize same-sex marriage. Their identification with Israel has drawn them into foreign policy … .
The Devil’s Sourdough and the Decline of Nations , by “Spengler”. (See also Why Europe Chooses Extinction, Death by Secularism: Some Statistical Evidence, Power and the Evangelical Womb and It’s the Culture, Stupid.)