Patriotism is the last refuge of scoundrels- a saying
Anyway I am denying the fact 'Thailand was not dominated by Indian thought commerce and religion. Not only Thailand but entire East Asia where Buddhism spread about 1500 to 2500 years back was obviously under Indian influence {Buddhism originated in India and it was Indians who spread it in the rest of world}. India is an ancient civilization and that to a prosperous one so obviously its thought would spread far and wide and India was a center of trade a primary reason why Europeans were so eager to find a sea route to India. Every point of view of yours has been accepted but this does not mean that Thailand is India.
India in itself was a land of incredible spiritual thought so not only east Asia but almost any place on this planet where Indians have gone has come under some sort of spiritual influence so will it mean that land is part of India.
The land where present day India is located has always been called India but you just cannot define what the hell is India. Several enlightened men have tried to invent a definition of who is an Indian and what is India? But unfortunately they have all failed
Religion - Nah India has itself given birth to three major religion and innumerable of less famous sects that religion by itself just cannot explain the concept of belonging to India namely
People belonging to other religion also live here
People who speak different languages call themselves Indian so language will never be able define what is Indian
People of deferent ethnicity form today's India
And so on..
The fact that you want to add entire area where Indians once had an influence would be hazardous because India has a long history and Indian sphere of influence has spread far and wide.
Chinese people have been gobbling up India for centuries.
My experience is that Indians have an elemental if not visceral antipathy toward Chinese people, whether taken one at a time or as a nation.
Then there's the Mongol problem ~ WOW!!!!
From the Afghan point of view the only good thing they left behind was mondoo! Fortunately we have a neighborhood restaurant that specialize in Mondoo and Peruvian style charcoal roasted chicken.