Without entering (and winning) some kind of major conflict, this was always going to happen. Two things are important to think about. 1. Power should be measured in relative terms not absolute. (For a good treatise on this, read Paul Kennedy's Rise and Fall of the Great Powers ). The US would have to be keeping up with China, India, and rest of world to maintain its previous pole position. 2. Great nations/empires ge…
There is an unspoken alternative for dealing with being out-populated by rival states. That is to reduce their populations through sanctions, blockades, starvation and outright war. These methods have been used by the Soviet Union in Ukraine, Germany/USSR in Poland, Imperial Japan in China, Belgium in the Congo. It continues to happen today, most openly in Palestine.
Speaking as Indian, Our population growth happened when we were poor, and masses were uneducated/illiterate.
I have one more theory that- Population growth happens when poor people have access to lots of carbohydrates. Plus having lots of children is somewhat akin to having meat robot automatons whom you can send for physical work and make money.
If you want to use sanctions to do reduce population it doesn't work in the modern era. Poor people eat well and make babies.