March 12 is the 71st day of the year (72nd in leap years) in the Gregorian calendar; 294 days remain until the end of the year.
538 – Vitiges, king of the Ostrogoths ends his siege of Rome and retreats to Ravenna, leaving the city to the victorious Byzantines.
1088 – Election of Urban II as the 159th Pope of the Catholic Church. He is best known for initiating the Crusades.
1158 – German city Munich (München) is first mentioned as forum apud Munichen in the Augsburg arbitration by Holy Roman Emperor Friedrich I.
1579 – Start of the Siege of Maastricht, part of the Eighty Years' War.
1622 – Ignatius of Loyola and Francis Xavier are canonized by the Roman Catholic Church.
1689 – James II of England landed at Kinsale, starting the Williamite War in Ireland.
1912 – The Girl Guides (later renamed the Girl Scouts of the USA) are founded in the United States.
1918 – Moscow becomes the capital of Russia after St Petersburg held this status for most of the period since 1713.
1928 – In California, the St. Francis Dam fails; the resulting floods kill 431 people.
1930 – Mahatma Gandhi begins a 200mile (320 km) march to the sea to protest the British monopoly on salt in India.
1933 – Great Depression: Franklin D. Roosevelt addresses the nation for the first time as President of the United States. This is also the first of his 'fireside chats'.