Interesting, but it fails.
Had Texas not purposely be willing to annexed itself into U.S.A., I'd bet only either Texas or Mexico would be standing-but not both. But most likely it'd be Texas, because either or both the Union or Confederates would aid Texas, seeing as Mexico being a bigger threat. Thus, I think Texas (or Mexico) would own territory of current-day Mexico, Texas, and the entire south-west.
Then... later, Pearl Harbor would never be owned by the Americans. Alaska too. Japan would've been able to touch-down in present-day California with their superior fighting techniques, conquer Texas/Mexico. The Union and Confederates would be like "wtf?" and wonder why Texas/Mexico-the strongest nation they know of-got it's ass kicked. Then, with Japan realizing they have the most profitable mines in the world (Californian gold mines) would use the gold to advance themselves beyond Germany's power. Hitler realizes this, offers Europe a cease-fire (but easy enough to get out of) to get rid of Japan, including Stalin. With Japan ahead of everyone technologically, they eventually launch a few nukes-hitting Germany, England, and Russia.
So everyone calls on a cease-fire after the aftermath, Japan owning most of Asia and North America reluctantly agree. Afterwards, England takes this as a chance to try to take back the colonies in North America. Their reason for it? "We need to grow stronger to stop Japan from further attacks."
But the Union and Confederates have a different idea. They ally themselves then accept refugees from the Mexican/Texan lands, so long as the refugees that're able to fight are willing to fight. Canada still under control of the queen does nothing. Eventually, an uprising occurs. The Union/Confederate alliance makes war with Japan, using guerilla tactics to clean them out of the main-lands. Europe takes this as a chance to fight back against Japan again, since they'll be fighting on all sides. Japan's strength diminishes, but with their last breath they go suicidal, detonating their final nuclear bomb in their home land. Russia and Germany go to war afterwards, becoming similar to the cold war of our time. England decides to invade America, wanting their colonies back.
At least that's my general idea.