It was never Spain's, either, unless you buy the authority of the Pope to divide the world. Spain's claim to North America went all the way to the pole, which was what Pope Alexander VI gave them under the Treaty of Tordesillas and later the Treaty of Zaragosa.
Oddly, I don't think the Anglican Protestant British settlers who founded the United States or their forebears in Anglican England gave a rat's butt about what the Spaniard King and his Pope thought they "owned". The claim was laughable even when it was made.
This website also has some interesting tidbits: Cal Republic History . Fascinating that Revere's grandson raised the U.S. flag for the first time in Sonoma.
Thanks for the links, more reading!
The Pope's action was not so much to 'give' Spain N. America, but to help settle a serious issue between Spain and Portugal. Spain's 'sphere of influence' was to be N.Am. while Portugal's was to be S.Am, thereby reducing the chance of war.