The "Euro Disney Resort" officially opens with its theme park ''Euro Disneyland''. The resort and its park's name were subsequently changed to Disneyland Paris.
The USS "Samuel B. Roberts" (FFG-58) strikes a naval mine in the Persian Gulf, while deployed on Operation Earnest Will, during the Tanker War phase of the Iran–Iraq War.
Eric Peugeot, the youngest son of the founder of the Peugeot Corporation, is kidnapped in Paris. Then, he is released on April 15 in exchange for $300,000 in ransom.
American Civil War ndash Andrew's Raid - Union volunteers steal a Confederate locomotive, setting off The Great Locomotive Chase, famously involving the use of "The General" steam locomotive, which still exists in the 21st century.
Friday, April 12, 1861
The American Civil War begins with the bombardment of Fort Sumter, South Carolina.
Margaret Porteous is the first person recorded to die in the Great Plague of London. This last major outbreak of Bubonic plague in the British Isles was possibly introduced by Dutch prisoners of war. Two-thirds of Londoners leave the city, but over 68,000 die. Plague spreads to Derby.
Sunday, April 12, 1654
Oliver Cromwell creates a union between England and Scotland, with Scottish representation in the Parliament of England.ref name=Cassell's Chronology/
King Mangrai the Great of Ngoenyang established a new capital by founding Chiangmai and founded the Mangrai Dynasty that ruled the Lanna Kingdom of Chiangmai from 1296 to 1578.
Karl Sverkersson is murdered by Knut Eriksson (son of Eric Jedvardsson), who proclaims himself king of Sweden. However, Karl's half brothers Kol and Boleslaw proclaim themselves kings of Östergötland, in opposition to Knut, which leads to fights for the power in Sweden until 1173.
Emperor Leo I has his general Anthemius elected emperor of the Western Roman Empire. He allies himself with Ricimer, "de facto" ruler of Rome, and marries his daughter Alypia to him. This to strengthen the relationship and end the hostilities between the Eastern and Western Empire.