Đất nước = land & water, the native Vietnamese word for nation/country prior to adopting the Chinese quốc gia. Nước by itself can mean nation/country while đất by itself can mean "territory".
I asked my friend who is a native Vietnamese speaker. "nước" by itself without any other words to add context just means "water".
If you say "nước Việt Nam" it's obvious you mean the country. "đất" is used more for emphasis and clarity here. After all "nước này" could be "this water" ;)
Fair enough. I think using "colony" later on makes it obvious that we're not talking about water, but I can see why we would want to make the distinction at the beginning of the sentence.