"Delenda Est" is a short story written by Poul Anderson, part of his Time Patrol (1960) series. The title alludes to the Latin phrase Carthago delenda est ("Carthage must be destroyed") from the Third Punic War.
Renegade time travelers meddle in the outcome of the Second Punic War, bringing about the premature deaths of Publius Cornelius Scipio and Scipio Africanus at the Battle of Ticinus in 218 BC, and thus creating a new timeline in which Hannibal destroys Rome in 210 BC. This meant that western European civilization came to be based on a Celtic-Carthaginian cultural synthesis (rather than Greco-Roman, as in actual history). This civilization discovered the western hemisphere, and created certain inventions (such as the steam engine) long before the corresponding events happened in actual history (partly since there was nothing corresponding to the fall of the Roman Empire), but overall technological progress has been slow, since most developments are arrived at through ad hoc tinkering (there is no scientific methodology of empirically testing rigorous theories).
At the time of the story, Britain (Brittys), Ireland, France (Gallia) and Spain (Celtan) are under Celtic control, and the Celts also colonised North America, known as Affalon in this timeline. Italy (Cimmeria) is under Germanic domination, Switzerland and Austria exist within Helvetia, Lithuania (Littorn) controls Scandinavia, northern Germany and much of Eastern Europe, while a Carthaginian successor empire (Carthagalann) dominates much of Northern Africa. The Han (Chinese) Empire controls China and Taiwan, as well as encompassing Korea, Japan and western Siberia. Punjab comprises western India, Pakistan and Afghanistan.
The major global powers are Hinduraj, centered on India but also encompassing Southeast Asia, Indonesia and Australasia, and Huy Braseal, which controls much of South America. Technology is at roughly a 19th-century level, and transport is reliant on the steam engine. Christianity, Judaism and Islam do not exist in this polytheistic world. There is greater gender equality in this world, but slavery has also survived — though it is not connected with any race or ethnicity.
Manse Everard, 20th-century Time Patrol agent, finds himself in the new timeline, in Catavellaunan (approximately New York), facing a moral dilemma: Should he return to the past before the events that led to Carthaginian victory, and restore his original timeline by destroying the new alternative timeline, thus ensuring that its inhabitants will never have existed?
It is found in Guardians of Time and Worlds of Maybe edited by Robert Silverberg.