Short answers, pulled from the story.
The political split began in 632 when a gathering at Saqifa elected Abu Bakr as caliph instead of Ali ibn Abi Talib. This event created a division that defined centuries of Islamic history.
Approximately 72 family members and followers of Husayn ibn Ali were killed by an army loyal to Yazid near Karbala. The battle is cited as the definitive separation between Shia and Sunni sects and occurred on the tenth day of Muharram, now commemorated as the Day of Ashura.
Twelvers follow twelve divinely ordained leaders starting with Ali as the first Imam and continuing through male descendants of Muhammad via his daughter Fatimah. Ja'far al-Sadiq serves as the sixth Imam for this branch known as Imami or Ja'fari.
Shia Muslims form a majority population in Iran, Iraq, and Azerbaijan. About half of Bahrain's citizen population also identifies as Shia.
Shah Ismail I proclaimed Twelver Shia Islam as the official state religion of Iran in 1501. This decision caused sectarian tensions across the Middle East and led to conflicts such as the Ottoman conquest of Iraq in 1533.