Star Trek: 10 Starfleet Decisions With The Most Devastating Consequences

Prompted by a change in government on Cardassia, the Klingon Empire, unknowingly at the behest of a Changeling in the shape of General Martok, begins an invasion of Cardassian space. Their main drive is the belief that this shift couldn't have been achieved without Dominion interference. Chancellor Gowron, in his long-running quest for glory, played right into the Founders' goo, committing an enormous force to the assault.

At the time, Starfleet had a peace treaty with Cardassia. Therefore, it refused to join the Klingons in the invasion, once it became aware of their plans. Further to this, they officially condemned the assault. As a direct result of this, Gowron tore up the Khitomer Accords.

This ended the alliance between the Federation and the Klingon Empire. The first shots were fired as the Defiant flew to the rescue of Gul Dukat and the Detapa Council, though hostilities truly broke out with the Klingon assault on Deep Space Nine. Thankfully for the inhabitants of the station, they had been fortifying for a Dominion assault for almost an entire year, and they survived. There followed several months of ongoing attacks by the Klingons against Starfleet, resulting in the loss of many lives and starships, including the USS Farragut.