During the period of WW1 the UK was still firmly in control of India. Why didn't it take much advantage of its enormous population to flood the field with an almost endless supply of troops on its side?
British India in 1907 had aprox. 290 million people, Germany at that same time 67 million, the UK itself roughly 40 million.
Yes, the vast majority would not be up to fighting caliber but again the numbers sent in could have been so overwhelming that it could arguably have made-up for that fact.