India in and of itself was largely out of ww2, at the time the area was under british control, so it did technically declare war against germany and japan, but not much happened in/around india itself (except for a nationalist, anti-british army that was propped up by Japan, which is in and of itself a fascinating story, see the "indian national army")

However, indian troops were crucial in multiple operations, under british command during ww2.

1. in the CBI (china-burma-india) theater, multiple indian armies fought primarily in Burma, and were instrumental in pushing the japanese from the region.

2. in North Africa, there were multiple indian divisions that fought in the campaigns against Rommel, with the 8th division and a few independant brigades fighting at alemein.

3. in Italy, where they were perhaps most famous for their actions in capturing Monte Cassino.

as to why Indian involvement is not well know, most of the popular media about ww2 focuses on the northern european front (on the british american side from d-day on, and on the soviet side, from stalingrad on), and the american island hopping campaign against the japanese. with maybe the exception of the Flying tigers, very few are familiar with the chinese theater, or the campaigns in the middle east, western africa, norway, finland, etc..

Most of the Indian leaders were involved in the Quit India movement, where they refused to fight unless India was granted independence. However, the British Indian Army was formed by volunteers (becoming the largest volunteer army in history by the end, numbering over 2.5 million soldiers).

On the other side, Bose, a Congress leader disgruntled by what he saw as British hypocrisy in fighting against fascism while enslaving and exploiting India, allied with Japan and Germany in exchange for a promise of Indian independence. He formed the Indian National Army ,mostly made of POWs and volunteers, and the Provisional Government of Free India.