Answer :
"between the two countries"
A prepositional phrase needs to contain a preposition, in this case it would be "between". As well it needs to modify the sentence and it should mention the object of the sentence. So "between the two countries" is correct.
A prepositional phrase needs to contain a preposition, in this case it would be "between". As well it needs to modify the sentence and it should mention the object of the sentence. So "between the two countries" is correct.