Pod in the 1840s.
Negotiations between the USA and Great Britain over Oregon Country fail, and the USA and Great Britain go to war against each other in 1848. However, internal turmoil from the Revolutions of 1848 that somehow spread to Great Britain prevents Great Britain from doing much of the war. (Great Britain goes into another civil war.) As a result, the USA is able to get all of Oregon Country, all the way to 54-40. However, the USA does not get Vancouver Island. Also, the USA was also able to invade New Brunswick and Ontario. Further negotiations between the USA and Great Britain results in two thing:
1. the selling of the Northwest Frontier to the USA in order to pay off debts gained by fighting the civil war in Great Britain
2. the fixing of the Anglo-American border to be the way they are right now in America
In 1854, in order to raise money to help defeat a Swedish-Austrian-French-Ottoman alliance, Russia offers to sell the USA Alaska. The USA accepts, but Russia loses the war anyways.
In the Gadsden Purchase in 1853, the USA gets all of Mexico up to the border in the map. However, with abolitionists taking over the Whig Party and rising to the Presidency and taking both houses in 1856, The USA loses most of the land and more, because other states and territories have seceded too. One notable state that seceded from the USA is Maryland, forcing the USA government to relocate to a different place. In the end, the USA loses the war of secession, and a new country forms. It could be named anything, but just for familiarity's sake, I will call the new country the Confederacy.
Eventually, imperialists in both countries rise, and take part in partitioning the Caribbean and Central America amongst themselves.
A war erupts between the USA and the Confederacy and the UK, with the USA against the other two countries. The striped patterns in the map represents how far the British and Confederates have advanced against the USA.