At the outbreak of the Greek War of Independence, Kasos possessed a population of 7,000 people and a sizeable fleet. With the onset of hostilities, the Kasiots began raiding the coasts of Anatolia, the Levant, and even Egypt. Under their elected admiral, Nikolaos Gioulios, they also often gave aid to the other Greek rebels in Crete, Samos, and elsewhere. As George Finlay reports, "It was said that the Kasiots usually murdered their captives at sea", especially since the island was barren and could with difficulty sustain its existing population. Apostolos Vakalopoulos also points out that control of Kasos was necessary for consolidating Ottoman–Egyptian control over rebellious Crete; strategically, Kasos was analogous with Samos and Psara as an advanced bulwark and base of operations for the Greek revolutionaries.
Already in June 1823, an Egyptian fleet commanded by Ismael Gibraltar disembarked 3,000 Albanian troops under Djiritli Hüseyn Bey at Crete. By spring 1824, Hüseyn Bey, by a judicious mixture of force and terror, combined with amnesty towards the Greek rebel leaders who were willing to submit, had managed to wipe out the Greek revolt in Crete. With Crete subdued, the Ottoman sultan, Mahmud II, and his semi-autonomous governor in Egypt, Muhammad Ali, turned their attention on Kasos and resolved to make an example of the island.
The Kasiots learned of their intentions, sending letters to the Provisional Administration of Greece to request aid, but the government replied that it was unable to mobilize the fleet for lack of money. The Egyptian fleet first appeared off the island on 2 June, launched an ineffectual bombardment, and made moves as if to land troops; as this was probably merely a reconnaissance mission, it soon departed. It was not until 18 June that the full 45 ships of the Egyptian fleet appeared off the island.
The island was not well fortified, apart from a few guns placed at the coast to cover the likely landing place. The Egyptian commanders furthermore contrived to deceive the islanders: after sailing past the island exchanging heavy artillery fire for two days with the defenders—on the 19th alone the Egyptians fired over 4,000 shots—the fleet moved towards the northern tip of the island. There it launched 18 great boats, pretending that it would make landing there, covered by much musket fire; while 24 boats with 1,500 Albanians landed behind the village of Agia Marina under cover of night on 19 June. The bulk of the population lived in the four mountainous villages around the main town, which were now between two hostile forces. Hüseyn Bey issued a call for surrender, rejecting requests for time to consider the proposal. Finally, the village elders chose to submit; this did not prevent Hüseyn Bey from executing many of them later.
The men who were posted on the shore batteries on the western side, under a certain Captain Markos, put up firmer resistance, but they too were overcome, as the Albanians, veterans of mountain warfare, used the cover provided by the higher ground to approach and defeat them, suffering only 30 killed and wounded in the process. Markos himself was captured and brought bound before Hüseyn Bey, where suddenly he managed to break his bonds, grab a knife from one of his guards, and kill three of them before he too as killed. The Albanians were given 24 hours to plunder at will; the Egyptian troops seized much plunder, as well as 15 larger and forty smaller vessels, while three newly built ones were burned. After that, however, Ismael Gibraltar and Hüseyn Bey firmly restored order on the island, executing three Arabs who disobeyed orders to that effect.
500 Kasiot men were slain, but a general massacre was averted allegedly because the first Albanians to come ashore were Christians, who interceded with their Muslim fellows to spare most of the male population. However, over 2,000 women and children were sent to the slave markets of Egypt and Crete. The Egyptian admiral managed to recruit a considerable number of survivors—as well as from the neighbouring islands of Karpathos and Symi—as sailors into his own fleet, offering a salary of 50 kuruş a month, before returning in triumph to Alexandria with his captured vessels, as well as fifteen elders and the families of the principal Kasiots as hostages.
News of the danger facing Kasos arrived at Hydra on the 21st, and the Greek fleet mobilized with much delay and difficulty, not arriving before the island until 2 July. The agha left as governor by Ismael Gibraltar fled the island to Karpathos, but the island was entirely destroyed; Vice Admiral Georgios Sachtouris and his captains proposed to the islanders to transport them to the Peloponnese, but the Kasiots refused, and the Greek fleet, receiving news that the main Ottoman fleet had set sail from Constantinople, was forced to leave the island. Indeed, the destruction of Kasos was followed soon after by another, heavier blow for the Greeks: the Destruction of Psara.