​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 . I want to promote virtue and prevent vice.”
Ref.: Bihar Al-Anvar: V.44, P. 329.

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Imam Hussein (as) is the grandson of the Prophet of Islam(as) . Forty days have passed since his martyrdom.  And every year, his lovers from all over the world go to his shrine in Iraq….
And this year we are deprived  because of the spread of Corona. However, our hearts miss his 6-corner shrine…