Many ethnic groups migrated into Vietnamese territory at different historical periods: some came thousands of years ago, fairly early, and some hundreds of years ago. Regardless of that, once you have lived on the S-shaped strip of land*, you are all considered of the same roots and are compatriots united by mutual understanding, allegiance, attachment and care.
*Vietnam is a long, narrow nation shaped like the letter S.
Some hundreds of years ago: I think this is a case of 'verb ellipsis' but I've read some examples on google and are now really confused. I'm not sure if it truly is or if I'm doing it right.