Spain’s claims were a bit tenuous. Fourteen years later, they would evaporate in negotiations with Secretary of State John Quincy Adams. By this time, Spain would be embroiled in civil war, leaving it without troops to vigorously defend its claims. President James Monroe was more interested in acquiring Florida from Spain than the distant and, perhaps, worthless lands of the Pacific Northwest. Spain’s claim to Florida was practically ancient compared with its hold on the Western lands of North America. The country’s hold on Florida dated back to 1513 when Juan Ponce de Leon sailed to the peninsula, naming it La Florida, “land of flowers.”
Secretary Adams was adamant about the young country stretching from coast to coast, and negotiated much longer than the president would have preferred. Finally, on February 22, 1819, Spanish Minister Plenipotentiary to the United States Luis do Onis met with Secretary of State Adams to sign a treaty. They had agreed that Spain would give up Florida, but also that the two countries would settle on the 42nd Parallel west to the Pacific Ocean as the boundary between their territories.
Florida and Idaho have little else in common, but they did come under the dominion of the United States on the same day.
That 1819 treaty set the southern border of what would become Idaho and Oregon.
My source for this post is the very readable book called Inventing Idaho, by Keith C. Peterson. If you’re at all a student of Idaho history, it needs to be in your library.