Nuclear war.
It would not decrease the population, rather would wipe it. Ever heard of
Nuclear winter.
Any kind of war and disastrous kind of event that actually catches the attention of all kinds of group in the world, it will be an chain of event
War would only lead to a reduced male population which would be followed by a very low productivity and economic growth. Plus a major war would also take its toll on infrastructure which would be completely destroyed. Look at Russia's demographic which is still to improve the population dynamics after WW2.