The 2367 to 2368 struggle between Gowron and the Duras family for leadership of the Klingon Empire, with the Romulans secretly backing the Duras side.
The Klingon Civil War broke out after Gowron was named leader of the High Council. Lursa and B'Etor, the sisters of the dead Duras, put forward Duras's young illegitimate son Toral as a rival claimant. As Arbiter of Succession, Picard rejected Toral's claim, and the Duras family took up arms.
In the Next Generation two-part story Redemption, Worf resigns his Starfleet commission to fight for Gowron alongside his brother Kurn, and in return Gowron restores their family's honour. The Duras forces were secretly being supplied by the Romulans, led by Sela.
The Federation would not take sides openly, but Picard set up a blockade fleet along the border to expose the Romulan supply line. When Data's ship, the Sutherland, revealed the cloaked Romulan convoy, the Romulans withdrew and the Duras forces collapsed. Gowron won the war, and Worf chose to spare Toral and return to Starfleet.