Frieda and Fritz have seven kids and run an efficent but small farm. There really is no place in a depressed world to emigrate and prosper. Despite very hard work, they become poorer. Ivan and Lana also live on a commonly owned farm. They have seven kids as well. They live with eight other big families. The farm is much bigger than the Fritz farm but is run very inefficently and the families are chronically hungry and live in poverty. Eventually Fritz and his friends get a very bad idea.
This scenario recurs throughout history. The next big war will be between resource starved, populated Chima coveting the rich , underpopulated Russian Pacific territories and the Siberian treasure house.
China’s latest decision to allow more children may improve a near term problem, but only increase the likelihood that the move on Russia you describe will be more probable. Only hope for the rest of the world they do not go nuclear.