A series of conflicts caused by genetically engineered humans.
In the late twentieth century genetic engineering had reached a point where a group of scientists created a generation of enhanced humans. These Augments were stronger, smarter and more resilient than ordinary people, and many of them were also arrogant and ruthless, believing that their superiority entitled them to rule.
Several of them, led by Khan Noonien Singh, took control of large parts of the world and fought one another and their rivals in a series of conflicts known later as the Eugenics Wars. Millions of ordinary people suffered under their rule, and humanity eventually united to overthrow the tyrants, at great cost.
Khan and some of his followers escaped Earth aboard the sleeper ship Botany Bay, drifting through space for centuries until the Enterprise found them in the 2260s. Kirk's choice to exile them on Ceti Alpha V led, years later, to the events of the Mutara Nebula. The wars left a lasting fear of genetic engineering in human culture and led the Federation to ban it, a rule that shaped the story of Dr. Bashir.