Romeo and Juliet were two 24th-century AIs with the misfortune of being on opposite sides of an ideological spat within the UESC. The story goes that two influential families within it, the Montagues and the Capulets, began working to undermine each other, with their AIs and best agents (Mercutio and Tybalt, respectively) as their primary tools. Unfortunately for all involved, Romeo and Juliet had their own agendas.
The historical record of this incident is generally agreed to be complete in all but one area: what exactly became of the AIs at the end. To this day, it's considered one of the great unsolved mysteries of Sol Core.