The Invasion of Septimus III was a battle fought during the Dominion War. In 2375, the Federation Alliance were progressively wearing down the Dominion, pushing deeper into Cardassian territory. With their forces stretched thin, the Dominion began withdrawing from many Cardassian worlds deemed non-strategic, leaving them nearly defenseless against invasion. Septimus III was one of these worlds.

The Cardassian Eleventh Order's headquarters were located on this planet, though the Order was badly weakened from numerous battles throughout the Dominion War, and had been reduced to a reserve unit. General Martok claimed the Order consisted of "old men and walking wounded", and that the planet would fall with ease.

Legate Damar realized the Eleventh Order's dire situation and pleaded with Weyoun to send reinforcements. Weyoun promised he would 'deal with it'. However, Weyoun's reassurance was without merit. Within a matter of days, Martok landed fifteen divisions on the planet and successfully conquered Septimus III. The entire Eleventh Order, consisting of 500,000 loyal Cardassian soldiers, was wiped out.

When news of the massacre reached Dominion Headquarters on Cardassia Prime, Damar furiously confronted Weyoun, who justified the Dominion's inaction by claiming that doing so had forced the Klingons to commit valuable resources to conquer a "strategically worthless" planet. When Damar demanded to know how many more sacrifices the Cardassians would have to make, Weyoun coldly replied that the Dominion would make whatever sacrifices the Founders deemed necessary to win the war.

Weyoun's betrayal, as well as the secret alliance made with the Breen fueled Damar's rage to the boiling point. Damar subsequently decided to rebel against the Dominion, thus leading to the Cardassian Rebellion. (DS9: "Strange Bedfellows")

Cardassian losses at Septimus III were indeed staggering, as later in the same episode Damar stated that 7 million Cardassian soldiers had died in the entire war up to that point.
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