Leonine City
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The Leonine City (in Latin Civitas Leonina) is that part of the city of Rome around which Pope Leo IV commissioned the construction of a wall for military defense during the 9th century. It is on the opposite side of the Tiber from the seven hills of Rome. The Vatican City is within that area, but the Leonine City, containing also the Roman rione of Borgo, is much more extensive than the tiny Vatican City.
In 1870, when the military forces of the Kingdom of Italy overthrew what was left of the Papal States, the Italian government intended to allow the pope to keep the Leonine City as a small remnant Papal State. But Pope Pius IX would not agree to that arrangement, and thus there was a 59-year standoff, settled in 1929 by the Lateran Treaty, which recognized the sovereignty and independence of the Vatican City.