The PoD is the United States gets all the mainland of former British North America in the American Revolution. After that, pretty much everything goes differently.
Having to deal with the francophone population since the beginning, and a larger imbalance between the slave-free states ratio, the United States is not able to achieve the same success with Manifest Destiny as in OTL.
While they aquired the pacific northwest much more easily, lacking a strong rival to their claims in the region, the enterity of the southwest has remained under firm control of a comparatively more powerful Mexican government controlled by the Imperial House of Iturbide.
While the Mexican-American war still happened in the early 1850s, internal issues limited the United States government to a quick regional war with ended with the usurpation of a Texas which had it's initial dreams of independence crushed by the mexicans.
The tensions between south and north escalated quickly in the 1860s, and finally ended with the secession of several southern slave states as the Confederate States of America - opportunistic as they were, americans in the still unorganized Oregon Country formed their own independent government (which they intended it to be neutral), and the Quebecois similarly left the union
The Civil War between the USA and the CSA lasted seven years, after which the Oregon Country still refused to be reincorporated (alot of people having escaped from the east, fleeing from war); but as preparations were made for another military campaign against the pacific nation... the British, French, and Russians intervened diplomatically in the Oregon Country's favour - this did not stop the Americans, which attacked anyway, drawing the ire of the European nations.
Following this, and after rather agressive negotiations, Quebec re-joined the Union peacefully. This further angered the European empires, which were beginning to see the United States as a threat, and thus began to heavily invest in Mexico as a counterweight.
By the first decade of the XXth Century, the United States had become a Great Power. But unlike in OTL, the Monroe Doctrine never became mainstream, as (imperial) Mexico was instead seen as the natural leader of the Latin American countries, and as it was supported by the European powers it greatly limited the United States capacity to do anything.
In 1913, a skirmish between Spain and the United States escalated to what would be known as the Atlantic War or the European-American War. It was something that the american leaders had been preparing for since decades ago.
A coalition lead by the United Kingdom including Spain, France, Netherlands, Germany, and Russia fought across the atlantic and in the caribbean against the United States, which aimed to 'liberate' the european colonies in America.
The war ended in 1916. After three years of intense fighting in the atlantic, the blockade of the american atlantic forts, the occupation of Seattle by a combined Russo-British Task Force, a disastrous invasion of Cuba in which the american navy was decimated, and Mexico partially mobilizing and threatening to invade Texas...
it was clear that the United States had lost. The peace treaty was harsh, demanding reparations, and heavily restricting the size of the navy. The ports of Vancouver, New Orleans, and Miami would be occupied by the defending coalition until the reparations had been met.
This lead to the United States going extreme radical the following decades, voting an authoritarian militarist nationalist party in the 1930s.
Meanwhile, in Europe, the countries that had fought the United States form a sort of proto-European Union, except led my their respective monarchs and with an emphasis on military cooperation.
The two projects begin to clash and tensions flare when the United States suspend the payment of the reparations and rebuild their navy, and finally the World War begins in 1938 when the United States forcefully expels the occupation forces from their ports.
In a grand naval campaign, the United States occupies all the colonial posessions of the european powers in the americas, as well as strategical territories that belonged to weak neutral nations (such as the caribbean coast of Venezuela and Colombia, the Mosquito Coast, and the isthmus of Panama).
The United States' wanked navy also manages to push back the european navies to their side of the atlantic, and American expeditions land sucessfully in French West Africa, German Central Africa, and British South Africa.
Drunken with victory, in their anti-colonial crusade, Manifest Destiny reborn, the United States fleet also attacks Japanese Hawaii (British ally) to decimate their navy there and deny them the ability to invade the mainland, invades Russian Alaska (which is considered a Russian colony in the sacred American mainland), and begins the pre-emptive land invasion of imperial Mexico by crossing the Rio Grande and an additional campaign in California, while assaulting amphibiously the port of Veracruz.
However, by 1943 the tables are turned, and the americans have been expelled from Africa, the Russians have landed in Alaska to take it back and push into the U.S. proper, and the advance into Mexico has been stopped and reversed just a few miles from Mexico City.
Finally, the United States government dissolves in late 1945, after the european forces had destroyed the overextended american navy, taken their caribbean colonial posessions back, blockaded all atlantic ports, and landed in New England (which immediately changed sides) from where they pushed towards virginia, the ohio, and quebec (which also declared itself independent and allied itself with the europeans).
By this time, the Russians and Japanese had also taken control of the pacific northwest, and the mexicans had pushed almost to the missisipi.
By 1947, most of the midwest and the south had yet to surrender, still fighting a vicious guerilla against the occupation forces. However, by 1949, most of the continent had been pacified, and the ocupation forces had granted autonomy to the local governments, which resttructured themselves into the independent nations seen on the map.
Thus, the United States of America, once a might empire stretching from Hawaii to Africa, and from Alaska to Venezuela, had ceased to exist pretty much.