War can be good at killing people, but not in the numbers needed to bring the population back to what is healthy for the planet.
Historically, that had always been by disease much more than by war.
Case in point, the bloodiest war until the modern era was the Reconquista. And in that and the aftermath (including the Inquisitions", the death toll was only around 7 million people.
That is barely a drop in the bucket to the Black Death, which killed around 200 million. Or the European diseases that killed over 50 million Indians in the Americas in the 16th century. Even the death toll of all of WWII (just over 50 million) is nowhere near enough.
No, my bet is on disease. That is natures way of culling out excess numbers when starvation does not do it. And we see it in all species, not just humans. Populations get too high, most times the animals will starve. If that is not enough, normally disease follows.
We really did didge the bullet with COVID. It was about a virulent as the Spanish Flu was, but only caused a fraction of the number of deaths. If we had actually had a repeat of Spanish Flu, the total number of deaths would not have been the less than 7 million that we had, but over 70 million. And heaven help us if some viral version of
yersinia pestis ever makes an appearance. That disease literally wiped out half the population of the planet.
In 1300, the global population was around 360 million. By 1400 almost 50 years after bubonic plague had largely died off, that had finally risen to around 350 million. The only time in over 7,000 years of population numbers that there was a decrease from one decade to the next. Not even WWII did that, as even with as many had died in that conflict, the population was around 100 million more when it ended than when it began.