At the turn of the 2nd century BCE, the Second Punic War between Carthage and Rome had ended. Rome was eventually victorious, but had suffered some significant and bad defeats. The peace treaty was even tougher for Carthage – it stripped them of many of their territories, their wealth, and restricted their actions.
Fast forward 50 years later, there was another conflict between Carthage and Rome – this time in a Punic-turned-Roman-city called Massinissa. Cato, a famous Roman orator and senator, was sent to Massinissa to investigate. He had fought in the Second Punic War in his 20s. Cato was surprised to see that, since the end of the Second Punic War, Carthage had become a thriving and wealthy city again.
When Cato came to back to Rome, he called for the war against Carthage – a war to stop them once and for all. He ended his speech with the phrase:
Carthago delenda est.
Carthage must be destroyed.
Cato would go on to end every speech he gave with Carthago delenda est, even if the speech was on an unrelated topic. He would continue to advocate a final war against Carthage for years. In 146 BC, nearly 8 years after Cato ventured back to Carthage and saw its wealth, would Carthage attack Massinissa and give Rome a reason to star the Third (and final) Punic War.
Carthago delenda est has become somewhat of a rallying call against a common enemy – a call for total war. Zuckerberg used it in an internal speech at Meta when Snapchat and Google+ launched (which, at the time, were existential threats to Facebook). See Ben Horowitz's blog post on Peacetime CEO/Wartime CEO.