As bloody as the world wars were, they weren’t particularly bad by historical standards. The 17th century was uniquely bad for 2nd millennium AD, with Little Ice Age & glut of specie leading to state collapses & population decline across Eurasia.

Falls of Rome & Tang in 1st millennium were worse, but at least they were recorded. Records of the even worse Bronze Age Collapse around 1200 BC barely survived for some areas, & for other regions all we have are archaeological indications that they regressed to the stone age.

Further back in the 3rd millennium BC, an even worse series of catastrophes occurred - the Indo-European invasions - ending the Megalith Builder Civilization with their urban settlements & leaving much of Europe depopulated for 600 years.

By the time of their destruction the Megalith Builders themselves had been in a centuries long decline from their Golden Age in late 5th & early 4th millennium. Their great realms had likely disintegrated around 3500 BC into smaller chiefdoms engaging in endemic warfare.

The Megalith Builders themselves were the result of WHG chieftains overthrowing the decadent EEF chiefs like those of the Linear Ceramics around 4400 BC & subjugating an 1800 year old neolithic civilization. Possibly related to spread of copper-working.