With the outbreak of the War of the Third Coalition in 1803, Papadopoulos formed a battalion and an artillery command with twelve guns from the garrison of Kamenets, with which he went to Corfu (part of the Russian-dominated Septinsular Republic). There in 1804 he was engaged in organizing the Republic's regular army, the Greek Legion. In the same year he was promoted to major general. In 1804 he also published in Corfu, in Greek and Italian, a military manual for the instruction of the Greeks in modern warfare (Διδασκαλία στρατιωτική προς χρήσιν των Ελλήνων, "Military Teaching for the use of the Greeks"). In 1805, he published the Legion's regulations (Ερμηνεία της συνισταμένης Λεγεώνος των Ηπειρωτο-Σουλιωτών και Χιμαρο-Πελοποννησίων, "Explanation of the combined Legion of Epirote-Souliots and Himariot-Peloponnesians"), where he exhorted its readers to remember that they are descendants of the ancient Greeks, to emulate the deeds of the celebrated Pyrrhus and Skanderbeg, and bring new glory to the Greek name.