Short answers, pulled from the story.
The Burma Independence Army (BIA) was a Burmese force that allied with Japan during the invasion. It was officially founded in Bangkok on the 28th of December 1941, beginning with 227 Burmese and 74 Japanese personnel. Aung San, a student activist who had trained with the Japanese on Hainan Island alongside the Thirty Comrades, was central to its formation.
Japan invaded Burma primarily to cut the Burma Road supply route to Chinese Nationalist forces under Chiang Kai-shek, to secure Burma's own natural resources including oilfields at Yenangyaung, cobalt, and rice, and to protect the flank of its main offensive against Malaya and Singapore.
On the 23rd of February 1942, Major-General Jackie Smyth ordered the Sittang Bridge blown while most of the 17th Indian Division was still stranded on the Japanese-held side. The decision was made to prevent the Japanese from using the bridge to advance on Rangoon. Critics argued it stranded two brigades and delayed the Japanese capture of Rangoon by only ten days at most.
The Burma Army evacuated Rangoon on the 7th of March 1942 after implementing a scorched earth policy, destroying the port and blowing up the oil terminal. Japanese forces reached Rangoon on the 8th of March 1942, by which point the Burma Independence Army had grown to between 10,000 and 12,000 soldiers.
At least 500,000 civilian fugitives reached India during the retreat, while a conservatively estimated 10,000 to 50,000 died on the way. In the months after arrival, 70 to 80 percent of survivors were afflicted with diseases such as dysentery, smallpox, malaria, or cholera, with 30 percent described as desperately ill.
The Burma Road, completed in 1938, ran from Lashio at the end of a railway from Rangoon's port into the Chinese province of Yunnan. It was the primary route for moving aid and munitions to Chiang Kai-shek's Nationalist forces. Capturing Lashio and cutting the road was a central Japanese objective, achieved when the 56th Division swept through the Shan States.