Underworld is a movie featuring a population of Vampires and Werewolves at war with each other. Besides the vampire/lycanthrope war idea, the film and its sequel also feature the concepts of both species having a common ancestor as well as vampire/werewolf hybrids.
Lucian is a werewolf, and he is therefore not permitted to feel love towards any vampire. He works for them instead, so he works as he was raised to do, trying to keep his feelings for Sonja inside.
One day, everything went wrong and Lucian and Sonja were caught, Lucian is locked away and she burns to death in the sun for being pregnant with "an abomanation", that is the hybrid baby. This forces Lucian to go insane with revenge and he and the rest of the werewolves who didn't die trying to escape from the vampires reign, starting the war between Vampires and Lycans.
Approximately 800 years later the war is still raging and the vampire Selene meets Michael, someone who was in the wrong place at the wrong time. While Selene hunted Werewolves in the subway, Michael was bitten by one and Selene is now his care-taker despite the Lycan blood that now flows through his veins.