San Marino is a small republic situated on the Adriatic side of central Italy. According to tradition, St Marinus and a group of Christians settled there to escape persecution. By the 12th century San Marino had developed into a commune ruled by its own statutes and consul. Unsuccessful attempts were made to annex the republic to the papal states in the 18th century and when Napoleon invaded Italy in 1797 he respected the rights of the republic and even offered to extend its territories. In 1815 the Congress of Vienna recognized the independence of the republic. On 22 March 1862 San Marino concluded a treaty of friendship and co-operation, including a de facto customs union, with Italy, thus preserving its independence although it is completely surrounded by Italian territory.