On 7 December, the Japanese attacked the US naval base at Pearl Harbour and later declared war on Britain and the United States. In the following days, Japan invaded many European Colonies in East Asia including British territories of Hong Kong, Malaya, Singapore, and Burma.   

The British thought invasion of Burma Japanese land is unlikely so they neglected its defenses till they lost their position in Rangoon and its vital port had been lost. With Japanese moving north, the surviving Allied troops carried out a five-month battle in difficult terrain.

The battles are fought in a very brave manner by British and Indian Troops against the Japanese military facing adverse conditions who are believed to be invincible- 

Battle of Imphal https://www.india1914.com/Thebattleofimphal.aspx

Battle of Kohima https://www.india1914.com/thebattleofkohima.aspx