Betreka Nebula Incident
The Betreka Nebula Incident began an eighteen-year-long conflict between the Klingon Empire and the Cardassian Union. 

In 2372, Julian Bashir cited the incident to Elim Garak as an example of hostile relations between the two peoples, but Garak dismissed it as having been a "minor skirmish" that happened "ages ago." ()

See also
Klingon-Cardassian War, a conflict between the two powers from 2372-2373

External link 
 

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Conflicts