Final answer:
The sentence provided is a compound sentence because it contains two main clauses joined by the coordinating conjunction 'but'.
Step-by-step explanation:
The sentence in question is: A passport is not currently necessary for residents of the United States or Canada crossing the border, but one may be required in the near future. This sentence contains two main clauses joined by the coordinating conjunction 'but', which makes it a compound sentence. The two clauses 'A passport is not currently necessary for residents of the United States or Canada crossing the border' and 'one may be required in the near future' can stand alone as independent sentences, thus fulfilling the criteria for a compound sentence structure. There are no subordinate clauses present in this sentence.