N. Europe battle unknown for 3,000 years

From Science dot org

About 3200 years ago, two armies clashed at a river crossing near the Baltic Sea. The confrontation can't be found in any history books—the written word didn't become common in these parts for another 2000 years—but this was no skirmish between local clans. Thousands of warriors came together in a brutal struggle, perhaps fought on a single day, using weapons crafted from wood, flint, and bronze, a metal that was then the height of military technology. 

Struggling to find solid footing on the banks of the Tollense River, a narrow ribbon of water that flows through the marshes of northern Germany toward the Baltic Sea, the armies fought hand-to-hand, maiming and killing with war clubs, spears, swords, and knives. Bronze- and flint-tipped arrows were loosed at close range, piercing skulls and lodging deep into the bones of young men. Horses belonging to high-ranking warriors crumpled into the muck, fatally speared. Not everyone stood their ground in the melee: Some warriors broke and ran, and were struck down from behind.

When the fighting was through, hundreds lay dead, littering the swampy valley. Some bodies were stripped of their valuables and left bobbing in shallow ponds; others sank to the bottom, protected from plundering by a meter or two of water. Peat slowly settled over the bones. Within centuries, the entire battle was forgotten.

https://www.science.org/content/article/slaughter-bridge-uncovering-colossal-bronze-age-battle

Historians expressed surprise that a battle had occurred at all, much less one of that size, believing that wars were unknown before the establishment of towns and cities.

Equally surprising was the modern-day consideration of diversity within soldier ranks, DNA showing men from various parts of Europe fought, some whose old wounds on skeletons indicated mercenary professional soldiers.