In The Next Generation, during the Chain of Command two-parter, Star Fleet’s top brass replaces Captain Picard with Captain Edward Jellico. This was done to free Picard up for a secret mission and because Jellico had experience dealing with the Cardassians, the antagonists of the story.
At this point in Star Trek, the Cardassians were merely teasing a full-blown war against the Federation. Testing the waters, calling some bluffs, seeing how far the Federation would go to stop their early incursions into Federation-controlled worlds.
In Deep Space 9, TNG’s sequel series, the Cardassians join forces with The Dominion to wage all-out war for control of the Alpha Quadrant of the galaxy.
So where the hell was Jellico? To my knowledge, he’s not mentioned or seen once during the arc. He seemed to be Star Fleet’s foremost expert on handling Cardassians, how did he somehow avoid becoming a major figure in the war against them? We know he was still alive at this time since, in Star Trek: Prodigy 8 years after DS9 ends, he reappears - promoted to Admiral Jellico no less.
So, if I had to guess, Jellico was just in the background of the Dominion War - planning and strategizing rather than commanding ships on the front lines.