Operation Nightfall is a fictional operation that occurred on the Fox TV show 24. The failed mission took place exactly 2 years before the events of the first season.
The covert operation's goal, led by (then Delta Force) Jack Bauer and authorized by (then Senator) David Palmer, was to kill Victor Drazen, who had been in charge of "ethnic cleansing" campaigns and various other human atrocities in Bosnia and Serbia. Jack thought that the operation, during which all of his men were killed except him, was successful, and that Drazen was killed. During Day One, he realized Victor was alive. Bauer then realized that Day 1 amounted to a personal vendetta designed by Victor's sons Andre and Alexis, against Jack and Palmer for the events of Operation Nightfall. The Drazens claimed that Victor's wife and their daughter were killed in the blast during Operation Nightfall, hence the kidnapping of Teri and Kim Bauer on Day 1 -- the revenge was intended to be "an eye for an eye".