6. The Battle of Cannae
In 216 BC, Hannibal used a smart plan to trap and defeat a large Roman army at the Battle of Cannae. Most of the 86,000 Roman soldiers were killed or captured, which shocked the Roman Empire.
The loss made Romans fear the gods and led to human sacrifices. It took Rome 14 more years to finally win the war. This battle is known as a major defeat in history.