As population is one of the indicative of productivity (although a weak one), you can see how many big events were driven by population growth. For example, -

As population is one of the indicative of productivity (although a weak one), you can see how many big events were driven by population growth. For example, - During World War II, US had more people then Germany, France and UK combined. This certainly enabled deployment of massive armies on many front and huge amount of weapon production. - Japan's population suddenly started rising and overtook many western countries. Its increased productivity might be the reason why this tiny country felt it can take on the world. - India and China are odd balls. India had massive population since very early times compared to European countries. -Somewhere in 1870, US population crossed a threshold and became the most populated western country. -Population for 2100 AD is estimated at 10 billion.