The battle of Lake Vadimo (310 BC) was a major Roman victory that broke the power of the Etruscan cities involved in the short Etruscan War of 311/10-308.
The war began with an Etruscan attack on the city of Sutrium, on the border between Roman territory and Etruria. The Romans won two victories outside the city, but without breaking the siege. The consul Q. Fabius Maximus Rullianus then crossed the Ciminian forest, a wilderness that formed the border between Rome and Etruria, raided the Etruscan heartland, and probably won a victory at Perusia (this battle may have been fought at Sutrium).
In the aftermath of this defeat Perusia, Cortona and Arretium sued for peace, and were each granted a thirty year truce, but a number of Etruscan cities remained hostile, including V, in the upper Tiber valley. These Etruscan states were able to raise another large army, according to Livy the largest yet. This army was raising using a sacred law (lex sacrata), in which each man chose a comrade who was thus compelled to join the army.
This army clashed with the Romans at Lake Vadimo on the Tiber. This was a narrow battlefield, and probably forced the Romans to deploy their cavalry at the back of the army instead of on the wings. Livy provides a dramatic account of the battle. Both sides were so confident that they began the fight without the normal exchange of missile weapons. The Romans were forced to feed their second and third lines into the battle, until eventually the only reserves left were the cavalry. They dismounted and advanced across 'piles of armour and heaps of slain' to reach the fighting. By now the Etruscans were suffering as badly as the Romans, and the appearance of new Roman troops finally broke their will to resist and they turned and fled.
Despite claiming at the 'main strength' of the Etruscan army was left on the battlefield, Livy doesn't record any consequences following on from this defeat. Only after a fresh Roman campaign in the next year did the remaining Etruscan cities ask for peace, ending this short war.