Star Trek: 10 Things You Didn't Know About Garak
7. Shall I Count The Ways I Hate Thee?
From their first scene together, it was abundantly clear that Dukat and Garak loathed each other. The story as to why was drip-fed through the series, teased in such episodes as Cardassians, Civil Defence, and The Die Is Cast. Although audiences would discover that Garak was responsible for the death of Dukat's father, it falls to Beta Canon to fully explain why.
In Andrew Robinson's novel A Stitch In Time, Garak recounts an early mission for the Obsidian Order. He was tasked with apprehending Procul Dukat, who by then was suspected of having ties to a dissident group. He succeeded in catching him, leading to the brutal physical and mental torture of the old man.
Both the novel and the series confirm that, at one point, the elder Dukat trusted Garak. According to the novel, this was simply a ruse - Dukat believed Garak to be his own son, the younger Dukat, enough to leak certain information.
This was enough to put the man forward to trial, after which he was sentenced to death. Though none of this was shown on screen, it further develops both men's backstories, as well as the political atmosphere of Cardassia itself.