The Chinese launched a campaign against the four pests of rats, flies, mosquitoes and sparrows.

The sparrows were thought to be eating a lot of grain making the peasants hungry so the communists launched a campaign to destroy them, going after their nests and hunting them etc.

Well they killed a ton of birds which led to an imbalance in the ecosystem with insects thriving as their predator - sparrows - was thinned out.

So they had a locust plague and lost way more crops leading to huge famine and way more deaths..