Complex sentence: Spanish rights in the area were not conceded to the United States until 1818, although theoretically Spain had no exclusive claim to sell the region, which was the position of the United States, leading to a dispute with Britain known as the Oregon boundary dispute.
Split into simpler sentences: Spanish rights in the area were not conceded to the United States until 1818, although theoretically Spain had no exclusive claim to sell the region. Exclusive ownership was nonetheless the position of the United States in interpreting the rights it had bought from Spain, leading to a dispute with Britain known as the Oregon boundary dispute.