Tahmasp I (Persian: شاه تهماسب یکم‎‎; Azerbaijani: Şah I Təhmasib) (22 February 1514 – 14 May 1576) was an influential Shah of Iran, who enjoyed the longest reign of any member of the Safavid dynasty. He was the son and successor of Ismail I.

He came to the throne aged ten in 1524 and came under the control of the Qizilbash who formed the backbone of the Safavid Empire. The Qizilbash leaders fought among themselves for the right to be regents over Tahmasp, and by doing so held most of the effective power in hands in the empire. Upon adulthood, however, Tahmasp was able to reassert the power of the Shah and control the tribesmen with the start of the introduction of large amounts of Caucasian elements, effectively and purposefully creating a new layer in Iranian society, solely composed of ethnic Caucasians. This new layer, also called the third force in some of the modern day sources, would be solely composed of hundreds of thousands of ethnic Circassians, Georgians and Armenians, and they would continue to play a crucial role in Persia's royal household, harems, civil and military administration, as well as in all other thinkable and available positions for centuries after Tahmasp, and they would eventually fully eliminate the effective power of the Qizilbash in most of the functioning posts of the empire, by which they would also become the most dominant class in the meritocratic Safavid kingdom as well. One of his most notable successors, the greatest Safavid emperor, Abbas I (also known as Abbas the Great) would fully implement and finalize this policy and the creation of this new layer in Iranian society.

Tahmasp's reign was marked by foreign threats, primarily from the Safavid's arch rival, the Ottomans, and the Uzbeks in the far east. In 1555, however, he regularized relations with the Ottoman Empire through the Peace of Amasya. By this treaty historical Armenia and Georgia were divided equally between the two, the Ottoman Empire obtained most of Iraq, including Baghdad, which gave them access to the Persian Gulf, while the Persians retained their former capital Tabriz and all their other north-western territories in the Caucasus (Dagestan, Azerbaijan) and as they were prior to the wars. The frontier thus established ran across the mountains dividing eastern and western Georgia (under native vassal princes), through Armenia, and via the western slopes of the Zagros down to the Persian Gulf. The Ottomans, further, gave permission for Persian pilgrims to go to the holy places of Mecca and Medina as well as to the Shia sites of pilgrimages in Iraq. This peace lasted for 30 years, until it was broken in the time of Shah Mohammed Khodabanda.

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