When did the Jallianwala Bagh tragedy occur? What did General Dyer do without warning people not to hold a public meeting?
OR
Which tragic incident prompted Gandhi to launch the Non-Cooperation Movement?

The Jallianwala Bagh tragedy occurred on 13 April, 1919.
General Dyer without any warning opened fire upon the peaceful crowd that had gathered at Jallianwala Bagh and continued firing on them until all his ammunition was exhausted.