It's a common theme, especially in science fiction and fantasy works. An enemy group becomes involved with an ally group through some sort of conflict or adversarial relationship, and then they start falling in love with each other. For example, in Star Wars, the Empire has taken over the galaxy, but it turns out that many of its leaders were members of the Rebel Alliance before they were captured and forced to serve the Empire. Similarly, the Jedi were an ancient order of warriors who were responsible for maintaining peace and justice in the galaxy until they were destroyed by the Sith. Even characters within the same story or movie can be enemies one moment and friends the next. This is called "morality play" or "moral ground". In Shakespearean drama, all characters are given a background history for each scene they appear in, so that the audience can understand why certain actions are deemed acceptable or not acceptable by society at large. In literature, enemies can become allies through a process called "bonding", which means that they come to respect and like each other despite their differences.