Short answers, pulled from the story.
General Joseph Stilwell planned and directed the campaign against Myitkyina, while Wei Lihuang commanded the Chinese Expeditionary Force that carried out much of the fighting. Sun Li-Jen served as the second commander of the Chinese New 1st Army under that force.
The siege of Myitkyina took place during the Burma campaign of World War II, running from Stilwell's order to advance on the 15th of March through Genzo Mizukami's order to abandon the town on the 3rd of August. Key moments included the main assault on the airstrip on the 17th of May and the capture of the northern airfield on the 26th of July.
Myitkyina mattered because of its airstrip, which Stilwell called strategically vital as a source of supplies and aerial support in the difficult jungle fighting of the China-Burma-India Theater. Capturing the town and its airfield also allowed the Allies to open the Ledo Road, connecting the old Burma Road with China.
The siege ended on the 3rd of August, when General Genzo Mizukami ordered the Japanese garrison to abandon Myitkyina and then took his own life in literal compliance with orders to defend Myitkyina to the death. Chinese and American forces then cleared the city and surrounding area of remaining Japanese troops.
Allied forces included the Chinese Expeditionary Force's New 1st Army, the US Army's Merrill's Marauders, British Chindits under Mike Calvert, and elements of Francis Festing's 36th Division. Japanese defenders belonged to the 33rd Army under General Hondo Masaki.
The Chinese Expeditionary Force suffered such heavy casualties from combat, supply shortages, difficult terrain, and disease that it effectively ceased to exist as a fighting force and was disbanded. Its losses were the highest of any battle during the Chinese intervention in the Burma campaign.