Say the 13 colonies never break away, and instead get representatives in Parliament, while the Spanish Empire also reforms and survives. It is now 1940. The border is somewhere around the pre-1844 US/Mexican border. London remains the capital, but is first among equals rather than preeminent; its equals include New York, Philadelphia, Boston, Buffalo, Milwaulkee, St Louis, New Orleans, etc. Similarly, Madrid remains the Spanish capital, but the 1st city in the empire is far and away Mexico City, followed by Buenos Aires and Lima.
Now, on June 1, 1940, for reasons that you can make up, the Spanish Empire has declared war on the British Empire. What happens? How would you conduct such a war? Is such a scenario possible? Does this thread belong somewhere else? Thoughts?