What allowed Muslim forces to successfully conquer parts of India?
The fighting between the Gupta nobles allowed Muslim forces to successfully conquer parts of India. For more information, check out http://www.ask.com/wiki/Muslim_conquest_in_the_Indian_subcontinent.
Infighting between Indian Gupta nobles created enough instability for Muslim armies to invade India. Muslims still make up a significant minority in India, particularly in the north.
The dissolution from within of the Gupta armies is the likely force which allowed the Huna Muslim forces to conquer parts of India. Internal fighting within the Gupta may be to blame.