ostan (province), northwestern Iran, bounded by the Caspian Sea on the north, Azerbaijan on the northeast, Azarbaijan-e Sharqi ostan on the west, Zanjan ostan on the southwest, Markazi (Tehran) ostan on the south, and Mazandaran ostan on the east. Gilan was within the sphere of influence of the successive Achaemenid, Seleucid, Parthian, and Sasanian empires that ruled Iran until the 7th century AD. The subsequent Arab conquest of Iran led to the rise of many local dynasties, and Gilan acquired an independent status that continued until 1567. Gilan is mentioned in The Regions of the World, a Persian geography of 982. During the rule (1073-92) of Malik-Shah, the third Seljuq sultan, a secret sect of Ismailis who formed the core of Shi'ah resistance against the Seljuqs was established in Gilan and eventually became known as the Assassins. The Turko-Mongol invasions in the 13th century resulted in a huge influx of refugees, including the Qajars, into the sparsely populated region. The Qajars helped the Safavid rulers seize power at the beginning of the 16th century. Later, the Qajars sided with Nader Shah in 1736, when he was threatened by the Afghans. The Qajars rose as a dynastic power in 1796, displacing the Zand dynasty. Olearius, an 18th-century traveller, mentions the Gilan fisheries. In the 19th century, the rivalry between Russia and Great Britain in Iran took the form of economic intervention. After losing several battles with Russia, Iran was forced to grant economic and naval benefits to that country in Gilan, under the treaties of 1813 and 1828. In 1907 Great Britain and Russia divided Iran into three zones; the northernmost, including the province of Gilan, was the Russian zone. These events led to the growth of nationalist movements, and contingents recruited from Tabriz, Gilan, and Isfahan liberated Tehran, with the result that Ahmad Mirza, son of Shah Mohammad Ali, was proclaimed ruler. In the aftermath of the Russian Revolution, Bolshevik troops had control of almost the entire Iranian Caspian seacoast, and a separatist group proclaimed the short-lived Soviet Socialist Republic of Gilan. In the treaty of 1921 the Soviet Union handed back the region around Enzeli and gave Iran equal navigation rights on the Caspian Sea.
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