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The Quadrilatero (English: Quadrilateral) is the traditional name of a defensive system of the Austrian Empire in the Lombardy-Venetia, which connected the fortresses of Peschiera, Mantua, Legnago and Verona between the Mincio, the Po and Adige Rivers. In the period between the end of the Napoleonic Wars and the Revolutions of 1848, they were the only fully modernized and armed fortresses within the Empire.
The experience of the Second Italian Independence War of 1859, in which rifled guns had been used for the first time by the Italian Army, pushed the Austrians to build a second line of 8 forts, at c. 4 kilometers from the main line (completed in the spring of 1866), pivoting around Verona.
See also 
- Rothenburg, G. The Army of Francis Joseph. West Lafayette: Purdue University Press, 1976. p 18.