Answer by Balaji Viswanathan:
Contrary to popular myth, every part of India had its own mighty warrior classes. Valour is not regional. That is how you could have great empires emerging from every part of India. If Sikhs were the only martial group, we would have had the rest of India ruled by their empires. There were 18 major martial classes in British India:and much more before that.
How come there are more Sikhs in army and less Sikhs at the top? There is one theory that is used in a few places like this book:(pg. 49). Indian politicians have always feared the military's incursion into democracy and followed principles from British Raj to carefully engineer the army. One of the key principles is to keep one group of loyal minorities form the army and some other loyal minorities run the army leadership.
Of the first 20 Indian, there were two Kannadigas (Cariappa, Thimayya), two Tamils (Kumaramangalam, Sundarji), two Telugus (Shriganesh, Rao), two Gujaratis (Jadeja, Manekshaw), two Jammu Kashmiris (Malhotra, Vij), two Marathis (Rodrigues, Vaidya), two Bengalis (Chowdhuri and Roychowdhury), two Punjabis (Thapar, Raina) one each from MP (Bewoor), Kerala (Padmanaban), Uttarakhand (Joshi) and NWFP (Malik).
One thing we can see is the spread of regions – from Kashmir to Bengal to Gujarat to Kerala, although the army is far less heterogeneous. This way no one region gets too powerful when it comes to heading the Army. Next we see their tenures shorter [typically about 2 years vs 4 years in case of countries like the US] preventing them to become too settled. The third is that Army Chiefs are carefully selected from places [such as south India, Bengal or Kashmir] that would neither align with the bulk of army [drawn from Sikhs, Gorkhas in more numbers] nor align with the bulk of the population [political power in the Gangetic valley]. This seems too precise to have evolved on its own without some political engineering.
In the past 10 though it seems army leadership getting less engineered [finally Sikhs get their due credit at the top with two Chiefs of Staff] as the government finally stopped fearing the army, but instead sees a bigger role for the defense as India aspires to be a superpower.