This paper is a study of the military history of the Second Russo-Iranian war (1826-1828). The second war was sparked after two years of provocative activities by the Russian side which included border incursions in northern parts of Khanate of Iravan (Yerevan), beginning in 1815, and attempt at forcing the Iranian side to accept Russian diktat. The war occurred in two phases in 1826 and 1827, resulting in Iranian capitulation in February 1828 (Treaty of Turkmanchay). The paper examines the state of preparedness of each side and explains the reasons behind initial Iranian victories. It will be demonstrated that initial Iranian victories had more to do with lack of preparedness on the Russian side rather than any new and superior Iranian military capability. In this context, the battles of Lankaran (July 1826) and fall of that city to Crown Prince Abbas Mirza Qajar’s forces, and Ganjeh (in the same month) and the fall of that city will be examined. Then a detailed examination of the Battle of Shamkhor (September 1826) and Ganjeh (October 1826) will be undertaken. These two battles turned the tide of the war as Russian General Paskovich (Iravansky) took command. The main conclusion of the paper is that while the Qajar state managed to improve its military performance against Russian aggression, Abbas Mirza’s army was not prepared for this war, and his was posturing prior to July 1826 had more to do with attempting to bluff the Russians to back off. In this context, the Iranian side misinterpreted the developments in Russia (the Decemberist Rebellion), and viewed the Russian side not to be prepared to have an all-out war with Iran. The study is based on original Persian, English and French sources. Sources include both historiographies and memoirs and observations by Iranian and non-Iranian observers.