Citation needed, I don't think I've ever seen a historian make the argument that the determining factor in civilisational conflict was birth rates. Guns, germs, steel, and various other resources. Access to ruminants and horses, sure. But birth rates. That's a new one.
And now, thanks to increased control over the environment due to technology and science, there are new tools in our repository that our ancestors did not have, like actually sitting down and planning how to avoid an ecological collapse -- and waging war with advanced weapons that negate a pure "breed fast" strategy. It sure would be nice if we didn't have to return to regular periods of widespread starvation like they did back in the Middle Ages, when they were too uninformed to not remove all forests and powerless to avoid staying away from the carrying capacity of their world for any amount of time.
no need to conquer anybody — just fuck up the rest of the world and get net population increase through immigration from places where people don’t want to live