The Taiping Rebellion is a major event for 
China. It will start the civil war between China and 
Heavenly Kingdom. It can happen at any time when Chinese consciousness is above 3. It will very likely be above 3 for the most of the beginning of the game due westernization reforms. The civil war can only happen once.
It has a base mean time to happen of 12 months, which is increased to 120 months before 1850. On the other hand, the mean time to happen is halved to 6 months after 1850 and decreased to a miniscule 1.2 months after 1860. Heavenly Kingdom will gain all of its cores except Beijing and a war with the civil war casus belli on both sides will start immediately. The casus belli requires 85 warscore to enforce and will annex the other country completely. White peace is not possible.
Additionally all 
soldiers will gain 8 militancy and all POPs which are follow the 
Sunni religion or have one of the cultures Nanfaren, Zhuang or Yue will gain 5 militancy. All Pops in Heavenly Kingdoms capital Nanjing will however lose 5 consciousness and militancy.
When the event fires the player will have the option of choosing either The Chinese Empire or Heavenly Kingdom to fight as.
Chinas substates will not join the war on either side, and will remain substates of whoever wins the civil war.
The 1861 Scenario
At the beginning of the 1861 scenario, the war has already started with the civil war casus belli on both sides.