Short answers, pulled from the story.
The Jallianwala Bagh massacre occurred on the 13th of April 1919. A large crowd gathered at the location in Amritsar, Punjab to protest against the Rowlatt Act and the arrest of pro-Indian independence activists Saifuddin Kitchlew and Satyapal.
Brigadier-General Reginald Dyer ordered his troops to fire into the crowd without warning. He arrived at the Bagh with fifty troops armed with .303 Lee-Enfield bolt-action rifles and blocked all exits before beginning shooting toward dense sections where panicked crowds tried to leave.
Official records from the Hunter Commission confirmed deaths of 337 men, 41 boys and a six-week-old baby. The Sewa Samiti society independently investigated and reported 379 deaths plus 192 seriously wounded while Indian National Congress concluded over 1,500 casualties with approximately 1,000 killed.
Dyer stated later that the purpose was not to disperse the meeting but to punish Indians for disobedience. He intended to open fire if he found a crowd assembled and believed he had been confronted by a revolutionary army receiving approval from Major General William Beynon via telegram.
On the 13th of March 1940 Udham Singh shot and killed Michael O'Dwyer at Caxton Hall in London. Singh was an Indian independence activist from Sunam who believed O'Dwyer approved Dyer's actions and requested martial law imposed upon Amritsar granted by Viceroy Lord Chelmsford.