How India was once ruled: friend getting behind India (10)

'how india was' is the definition.
I can't tell whether this definition defines the answer.

'ruled friend getting behind india' is the wordplay.
'ruled' becomes 'colon' (I can't explain this - if you can you should give a lot more credence to this answer).
'friend' becomes 'ally' (an ally is a friend or supporter).
'getting behind' means one lot of letters go next to another (some letters go behind others).
'india' becomes 'i' (phonetic alphabet: alpha, bravo, charlie etc.).
'ally' after 'i' is 'ially'.
'colon'+'ially'='COLONIALLY'

'once' is the link.

Can you help me to learn more?