Complex sentence: Eventually the U.S. conceded to claim only as far west as the Sabine River, but Spain insisted upon the Arroyo Hondo boundary; the disputed region was known as Neutral Ground.
Split into simpler sentences: Eventually the U.S. conceded to claim only as far west as the Sabine River, but Spain insisted that the boundary should be upon the Arroyo Hondo. The disputed region was known as Neutral Ground.