Why did imam Hussain rise up on Ashura? why Ashura happened?

Imam Hussain (PBUH) explained his revolutionary goal as a will which he wrote to his brother, Muhammad bin Al-Hanifiah. The text in the above image is a part of Imam Hussain’s will:

“I did not rise up for the cause of wickedness, amusement, corruption or oppression, but I rose up to reform the Muslim community of my grandfather [Prophet Muhammad (PBUH)]. I want to promote virtue and prevent vice.”

Ref.: Bihar Al-Anvar: V.44, P. 329.