In 1803 Kostas Botsaris and the remnants of the Souliotes crossed over to the Ionian Islands, where they ultimately took service in the French-raised Albanian Regiment. In 1814, he joined the Greek patriotic society known as the Filiki Eteria. On the night of 21 August 1823 Kostas, under the leadership of his brother Markos participated in the celebrated attack on Karpenisi by 350 Souliotes, against around 1000 Ottoman troops who formed the vanguard of the army with which Mustai Pasha was advancing to reinforce the besiegers.