Map of "It Started Here", Directly before the Great War, or the People's War, depending on which side you were on. Taking place in 1912.
The POD here is in 1848. Martin Van Buren declines to run on the Free Soil ticket. This upset results in a loss for the Whigs, as historically Van Buren's influence ensured New York went for Taylor, giving him the election. The Democrats, now under Lewis Cass of course support the Compromise of 1850 and enforce the Fugitive Slave Act as per historically, leading the Whigs to surge in popularity in the 1852 election. Cass is renominated, and much like historically, the 1852 election comes to be a contest of personalities. Winfield Scott proves to be much more popular than Cass, and as result wins. The popular and physically massive military figure takes a more anti-slave stance, as the Whig Party was becoming the more "Anti-Slave" party at the time (until the Kansas-Nebraska Act). When the Kansas-Nebraska Act comes up this time, Scott vetos it, and thus "Bleeding Kansas" never grows to the out-of-control situation it does historically, with only very minor skirmishes. The 1856 election sees Scott ride to re-election, with the Republican Party never truly forming. By now the Whigs are generally the "Anti-Slave" party, while the Democrats are the "Pro-Slave" party. However, many Southern Whigs and Northern Democrats are unhappy with this state of affairs. Scott doesn't seek re-election in 1860, and the Democrats nominate James Buchanan and the Whigs nominate Millard Fillmore. A large segment of Southern Whigs and Northern Democrats thusly break away from the two parties, instead forming the "Constitutional Union Party", former Democrat Stephen Douglas is nominated, and successfully wins the election, if just barely. At the same time the Radical Democracy Party starts pulling a sizable portion of voters under John C Fremont, gaining a good chunk of votes due to it's proto-socialist ideals, as the Panic of 1857 had triggered it's creation, as well as it's extreme anti-slavery stance. During Douglas' presidency Immigration continues to flow into the U.S., and many feel the pressure as jobs become increasingly scarce for the poor immigrants on the West Coast. This is not helped by Douglas' inability to deal with the 1860 New Englander Shoemaker's strike, and the economic recession that occurred during in 1860. Nonetheless, America continued on. In 1861, hoping to distract the nation from the slavery issue, Douglas authorized William H. Seward to purchase Alaska in 1861. The Russian American Company, who's charter expired in 1861, was brushed aside by the Russian Tsar, and the United States soon acquired the new territory. However, this only raised more discontent, as the whole area was viewed as "nothing more than an Icebox". As the 1864 election rolled around, the Radical Democracy Party made an extremely well showing, winning many industrial centers, However, despite getting a plurality of the popular vote, and the majority of the electoral votes, it was not enough to win, and so the race was deadlocked between the four parties. When the case was brought up in the House of Representatives, the Whigs and Democrats, the Whigs being more fearful of the proto-socialist ideals of the RDP and the Democrats being more fearful of the radical anti-slavery ideals of the RDP, both unable to agree on voting for the other, eventually elected Douglas, despite him getting the lowest popular vote out of all 4 candidates. Outrage ensued as factory workers broke out all across the North in strike. Attempting to put them down, the Union Army was called in, but this merely made things worse. Full scale-civil war eventually broke out, with the Charismatic Alonzo G. Draper, one of the leaders of the New Englander Shoemaker's strike, taking the forefront. The United States rallied against them, yet many soldiers, especially conscripted immigrants found they had more in common with the workers, and would often join them. Soon much of the industry of the North was in rebel hands, and the rebels started moving South, burning down plantations and freeing slaves to join their cause as they went.
During this same period, France, Great Britain, and Spain would launch an invasion of Mexico, hoping to install Maximillian I as Emperor and set up a friendlier pro-European regime in the area. As Napoleon III had longer to think this plan over, he elected to try and keep Britain and Spain in the invasion force as long as possible. By 1867, much of Mexico was occupied, and the Workers had overrun the Southern portion of the United States, now threatening to overwhelm the US, as many former Free-Soilers had begun to rise in rebellion alongside the Workers. Mexican Liberals, pushed to the border of their country had begun to flee into the undefended territories of the United States, with France nearing complete control over Mexico, they decided to let slip their plans to both Britain and Spain, who withdrew from the Coalition. However, as much of the US military, especially in the stubborn area of Texas, started running into Mexico, the Revolution soon followed, the remaining Mexican Liberals joining forces alongside the Nearly victorious RDP, who would exile what remained of the US government to the newly purchased Alaska, only kept afloat with British support. As 1870 rolled around, and increasingly antagonizing, and newly United North German Confederation, began threatening France in Europe, and as result, the French abandoned Mexico, leaving the newly declared "Worker's Republic of America", a union between the Mexicans and Americans to be at peace.
Thus, the quote "It can't happen here", becomes "It started here".
I'll provide more write-ups regarding the delayed Franco-Prussian War, the Rise of Germany and Russia, the worries of Britain, and eventually the Great War, later on.
Feel free to ask questions now if your curious.