Klingon–Federation War (2372–2373)

In 2372, convinced that Cardassia's new civilian government had been infiltrated by Changelings, Chancellor Gowron launched a full-scale invasion of Cardassian space. When the Federation condemned the attack, Gowron withdrew from the Khitomer Accords, ending more than seventy years of alliance between the two powers.

Captain Benjamin Sisko's crew rescued the Cardassian Detapa Council and defended Deep Space 9 against a Klingon assault fleet, and Worf, who refused to side with Gowron, joined the station's staff. For about a year the former allies fought a bitter border war of raids, minefields and skirmishes, while suspicion grew that the conflict served only the Dominion. That suspicion proved right: General Martok, Gowron's trusted adviser, turned out to be a Changeling impostor.

The war ended in 2373 when Cardassia joined the Dominion. Facing a common and far more dangerous enemy, the Klingons and the Federation agreed a ceasefire and restored the Khitomer Accords, an alliance that would soon be tested in the Dominion War.