Detectives Vic Mackey and Dutch Wagenbach have had a heated rivalry for most of the series. Dutch considers Vic to be nothing more than a brutish thug. This in turn has led to Vic taking the rather public stance that he considers Dutch to be an over-educated elitist.
The two have engaged in several back-and-forth verbal fights over the course of the show, with Vic mocking Dutch's failure with women and Dutch briefly dating Vic's ex-wife Corrine. They very nearly came to blows in the fourth season when Dutch, who had just physically confronted another detective, accidentally hit Vic with a wild punch and was more than willing to escalate the confrontation before the entire incident was neutralized by Claudette.
However, there have been moments of mutual admiration between the two. Towards the end of season one, Dutch was able to successfully get a serial killer who was targeting local prostitutes to confess after a lengthy interrogation. This resulted in Vic conceding to Dutch that he is as smart as he claimed, and resulted in a truce between the two that lasted until the end of the third season.
Indeed, Dutch may be the only cop in the Barn who truly scares Vic, in terms of his potential to bring the Strike Team down if he ever put his mind towards the goal. This fear of Dutch going after the Strike Team was most notable during the money train heist arc, when Dutch began to close in on the Strike Team for their part in the heist. Vic openly showed fear that Dutch would not stop until he caught the Strike Team and tried to intimidate Dutch into dropping his investigation by threatening to resume his bullying of him, a move that effectively signaled the end to their truce.
Dutch's investigation of the money train ultimately triggered the tragic series of events that led to the death of Vic's close friend, Detective Curtis Lemansky. Dutch informed IAD Lt. Jon Kavanaugh about his suspicions, and Kavanaugh later used the information to trick Detective Shane Vendrell into thinking Lem had turned traitor against the Strike Team, causing Shane to murder his friend.
During Kavanaugh's investigation into the team, Dutch learned from Kavanaugh about his suspicion of Vic's involvement into Lemansky's death. Despite the dislike he held toward Vic, Dutch confided to Claudette that he doubted that Vic would murder a close friend, no matter the circumstances.